2. Hypervisors

The hypervisor is the piece that makes virtualization possible. If you know which types exist and how the processor helps them, you can choose the right product for each situation and fix the most common error when you create a VM: the missing 64-bit option.

How to work with this page

Estimated time: about 45 minutes of reading and 30 of practice.

1. Read sections 1 and 2, then build the stacks in the lab tab Hypervisor stack builder.
2. Read section 3 and follow the steps in the tab VT-x on/off.
3. Do Activity 2, where you compare real products.

1. What a hypervisor does

A hypervisor (also called a virtual machine monitor, VMM) is the software that creates virtual machines and runs them. It has three main jobs:

1. Share the CPU. It decides which guest uses each processor core and for how long.
2. Share the memory. It gives each guest its own memory area and keeps the guests apart, so that one cannot read the memory of another.
3. Present virtual devices. It shows each guest a virtual disk, a virtual network card, a virtual graphics card… and turns every request of the guest into an operation on the real hardware.

From the point of view of the guest, nothing special happens: it starts, finds a “computer” and runs.

2. Type 1 and Type 2

There are two families of hypervisors. The difference is what is under the hypervisor.

Type 1: bare metal

A Type 1 (or bare-metal) hypervisor starts directly on the hardware, just after the firmware (UEFI/BIOS). There is no normal operating system under it. It is a small, specialised system whose only job is to run virtual machines.

Applications    Applications
Guest OS        Guest OS
------------------------------
Hypervisor (Type 1)
------------------------------
Hardware

Because there are fewer layers, it is faster, more stable and has less attack surface. It is the normal choice in data centres and in the cloud. It usually offers advanced features: moving a running VM between servers (live migration), restarting VMs automatically if a server fails (high availability), and managing hundreds of VMs from one console.

Type 2: hosted

A Type 2 (or hosted) hypervisor is a normal program that you install on your usual operating system, like a browser or a text editor. The VMs it creates are windows on your desktop. When a guest writes to its disk, the hypervisor asks the host operating system to write a file.

Applications    Applications
Guest OS        Guest OS
------------------------------
Hypervisor (Type 2, a program)
------------------------------
Host OS (Windows, Linux, macOS)
------------------------------
Hardware

It is easy to install and you can keep working with your normal computer, so it is ideal for learning, testing and development. The price is a little more overhead, and one more layer to keep updated and secure. VirtualBox is a Type 2 hypervisor.

Comparison

Feature	Type 1 (bare metal)	Type 2 (hosted)
Runs on	The hardware	A host operating system
Performance	Closer to native	A bit lower, especially for disk and network
Installation	Dedicated machine, more planning	Like any program
Typical use	Servers, data centres, cloud	Laptops, classrooms, testing
Management	Many hosts and VMs, central tools	One computer, simple tools
Security surface	Smaller	Larger (host OS + hypervisor + guests)
Examples	VMware ESXi, Microsoft Hyper-V, KVM, Xen	VirtualBox, VMware Workstation, Parallels Desktop

Real products

Product	Type	Licence	Where you find it
VMware ESXi	1	Commercial (proprietary)	Company servers
Microsoft Hyper-V	1	Included with some editions of Windows (Pro, Enterprise, Education) and Windows Server	Windows servers and PCs
KVM	1	Free and open source (part of the Linux kernel)	Linux servers, cloud, and platforms such as Proxmox VE
Xen	1	Free and open source	Servers and some cloud providers
Oracle VirtualBox	2	Free and open source (some extras have a different licence)	Windows, Linux, macOS (Intel)
VMware Workstation	2	Proprietary (check the current terms of the owner)	Windows and Linux PCs
Parallels Desktop	2	Commercial (proprietary)	macOS

Why is KVM Type 1 if I use it from Linux?

You manage KVM from a normal Linux system, but KVM is a module of the Linux kernel itself: the kernel becomes the hypervisor. Hyper-V is similar: when it is enabled, Windows itself runs as a special guest on top of the hypervisor. For this reason both are considered Type 1.

Licences change

Companies change the names, prices and conditions of their products often. Before you choose a product for work, read the current licence on the official website.

3. Help from the processor: VT-x and AMD-V

The problem

Operating systems are written to run on real hardware. A normal OS kernel expects to run in the most privileged mode of the CPU and to control everything. But if a guest kernel could do that, it would take control of the whole computer and the hypervisor would lose its authority. The hypervisor must intercept the “dangerous” instructions of the guest.

There are three ways to solve this:

Method	How it works	Speed
Software virtualization	The hypervisor reads the guest code and rewrites or emulates the dangerous instructions before they run (binary translation).	Slowest, complex
Hardware-assisted virtualization	The processor has a special mode for guests. The guest kernel runs almost directly on the CPU, and the CPU hands control to the hypervisor only when a dangerous operation happens.	Fast
Paravirtualization	The guest knows that it is virtual and uses special drivers to talk to the hypervisor (for example, virtio network and disk drivers).	Fast for disk and network

VT-x, AMD-V, EPT and NPT

Modern processors have the instructions for hardware-assisted virtualization:

• Intel VT-x on Intel processors.
• AMD-V on AMD processors.
• EPT (Intel) and NPT (AMD) help to translate the memory addresses of the guest to real addresses without the hypervisor doing it by software. This makes memory access much faster.

Current versions of VirtualBox need hardware virtualization to work. In practice you will use a mix of the three methods: VT-x or AMD-V for the CPU, and paravirtualized (or very simple emulated) devices for network and disk.

Paravirtualization in VirtualBox

In the settings of a VM, System → Acceleration → Paravirtualization Interface lets you choose how the guest talks to the hypervisor. The Default option is a good choice for almost every guest.

Check and enable it

These features are often disabled in the firmware by default, mostly on laptops. The classic symptom is that VirtualBox only offers 32-bit versions in the list of guest systems, or says that VT-x is not available.

To check it, in Linux:

lscpu | grep -i virtualization
grep -Ec '(vmx|svm)' /proc/cpuinfo

The first command shows VT-x (Intel) or AMD-V. The second counts the processor entries that have the vmx flag (Intel) or svm flag (AMD). If the number is 0, the feature is absent or hidden. Remember that the output of these commands inside a guest may be different, because they show what the hypervisor lets the guest see.

In Windows you can open the Task Manager, tab Performance → CPU, and look for Virtualization: Enabled.

If it is disabled, restart the computer, enter the UEFI/BIOS setup and look for an option with a name like:

• Intel Virtualization Technology (Intel).
• SVM Mode (AMD).

Enable it, save and restart. The exact name and place change with every manufacturer.

Another hypervisor in the way

On Windows, features such as Hyper-V, Windows Sandbox or some security options use the hardware virtualization of the processor. VirtualBox can work together with them, but the VMs are often slower. If your VMs are very slow, ask your teacher before you change those features.

4. Choosing a hypervisor

Situation	Reasonable choice
Learning on a school or home PC	VirtualBox (Type 2, free)
You already have Windows Pro and need a simple option	Hyper-V
A company runs many critical servers	A Type 1: ESXi, Hyper-V on Windows Server, or KVM with a management platform
You use macOS and need good integration	Parallels Desktop or another Mac-specific product
You want to build a free virtualization server	KVM with a platform such as Proxmox VE

Containers are not VMs

You may hear about Docker and containers. A container shares the kernel of the host instead of running a full guest operating system. It starts faster and uses less memory, but it is less isolated than a VM. It is a different tool, and you will meet it in other modules.

Check what you have learned

1. What is the main difference between a Type 1 and a Type 2 hypervisor?

   Answer

   A Type 1 hypervisor runs directly on the hardware. A Type 2 hypervisor is a program that runs on top of a host operating system.

2. Classify these products as Type 1 or Type 2: VirtualBox, KVM, ESXi, Parallels Desktop.

   Answer

   Type 2: VirtualBox and Parallels Desktop. Type 1: KVM and ESXi.

3. You open VirtualBox and you can only create 32-bit guests. What is the most likely cause and the solution?

   Answer

   Hardware virtualization (Intel VT-x or AMD-V) is disabled in the UEFI/BIOS setup. Restart, enter the setup, enable Intel Virtualization Technology or SVM Mode, save and restart.

4. Why do Type 1 hypervisors usually have better performance than Type 2?

   Answer

   Because there is no host operating system between the hypervisor and the hardware. There are fewer layers, so less overhead and fewer things that can fail.

5. What is paravirtualization?

   Answer

   A method where the guest knows it is virtual and uses special drivers (for example, virtio) to communicate with the hypervisor directly. It avoids emulating real hardware and it is faster for disk and network.

Glossary

English	Castellano
hypervisor	hipervisor
Type 1 / bare metal	tipo 1 / sin sistema operativo anfitrión
Type 2 / hosted	tipo 2 / alojado
hardware-assisted virtualization	virtualización asistida por hardware
paravirtualization	paravirtualización
binary translation	traducción binaria
live migration	migración en vivo
high availability	alta disponibilidad
kernel	núcleo (del sistema operativo)
container	contenedor
firmware	firmware

Credits

This page adapts the SOM course notes by Moisés Mas Rocamora, Daniel Álvarez Santamaría and Jana Taboada León, licensed under CC BY-NC-SA 4.0. This page is shared under the same licence.