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

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