XEN

Xen is a virtualization system supporting both paravirtualization and hardware-assistant full virtualization. Xen is an open-source type-1 or baremetal hypervisor, which makes it possible to run  many instances of an operating system or indeed different operating systems in parallel on a single machine (or host). Xen is the only type-1 hypervisor that is available as open source. Xen is  used as the basis for a number of different commercial and open source applications, such as: server virtualization, Infrastructure as a Service (IaaS), desktop virtualization, security  applications, embedded and hardware appliances. 

Some main points on Xen:

• It named from neXt gENeration virtualization.
• Xen initially created by University of Cambridge Computer Laboratory.
• Xen is Open source (Licensed under GPL2).
• Xen offers high performance and secure architecture.
• Xen is powered by a growing and active community and a diverse range of products and services.

Virtualization in Xen

Paravirtualization:

# Uses a modified Linux Kernel (e.g. Linux-2.6.18-xen)

# Guest loads dom0's pygrub or dom0's kernel

# Front-end and back-end virtual device model

# Cannot run windows

# Guest "knows" it's a VM and tells the hypervisor

Hardware-assisted full virtualization: 

# Uses the same, normal, OS Kernel

# Guest contains grub and Kernel

# Normal device drivers

# Can run windows

# Guest doesn't "know" it's a VM, so the hardware manages it

Reasons to use Xen

Paravirtualization (PV)

# High performance (claim to fame)

# High scalability

# Uses a modified Operating System

Hardware-assisted full virtualization (HVM)

# Leading hardware vendors to enhance virtualization in x86 architecture

# Uses an unmodified Operating System