Xen

"Xen Project (pronounced as /ˈzɛn/ /prˈɒdʒɛkt/) is a hypervisor using a microkernel design, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently. It was developed by the Linux Foundation and is supported by Intel.

Xen Project runs in a more privileged CPU state than any other software on the machine.

Responsibilities of the hypervisor include memory management and CPU scheduling of all virtual machines ("domains"), and for launching the most privileged domain ("dom0") - the only virtual machine which by default has direct access to hardware. From the dom0 the hypervisor can be managed and unprivileged domains ("domU") can be launched.^[3]

The dom0 domain is typically a version of Linux or BSD. User domains may either be traditional operating systems, such as Microsoft Windows under which privileged instructions are provided by hardware virtualization instructions (if the host processor supports x86 virtualization, e.g., Intel VT-x and AMD-V),^[4] or para-virtualized operating systems whereby the operating system is aware that it is running inside a virtual machine, and so makes hypercalls directly, rather than issuing privileged instructions.

Xen Project boots from a bootloader such as GNU GRUB, and then usually loads a paravirtualized host operating system into the host domain (dom0).

Internet hosting service companies use hypervisors to provide virtual private servers.  Virtual machine monitors (also known as hypervisors) also often operate on mainframes and large servers running IBM, HP, and other systems.^{[citation needed]} Server virtualization can provide benefits such as:

• consolidation leading to increased utilization
• rapid provisioning
• dynamic fault tolerance against software failures (through rapid bootstrapping or rebooting)
• hardware fault tolerance (through migration of a virtual machine to different hardware)
• the ability to securely separate virtual operating systems
• the ability to support legacy software as well as new OS instances on the same computer

Xen's support for virtual machine live migration from one host to another allows workload balancing and the avoidance of downtime.

Virtualization also has benefits when working on development (including the development of operating systems): running the new system as a guest avoids the need to reboot the physical computer whenever a bug occurs. Sandboxed guest systems can also help in computer-security research, allowing study of the effects of some virus or worm without the possibility of compromising the host system."

https://en.wikipedia.org/wiki/Xen

https://wiki.xenproject.org/wiki/Main_Page