Xen Virtualization: Hypervisor (aka VMM aka Microkernel)

Xen is a virtual machine monitor for x86 that supports execution of  
multiple guest operating systems with unprecedented levels of  
performance and resource isolation.  
  
This package contains the Xen Hypervisor. (tm)  
  
Modern computers are sufficiently powerful to use virtualization to  
present the illusion of many smaller virtual machines (VMs), each  
running a separate operating system instance. Successful partitioning  
of a machine to support the concurrent execution of multiple operating  
systems poses several challenges. Firstly, virtual machines must be  
isolated from one another: It is not acceptable for the execution of  
one to adversely affect the performance of another. This is  
particularly true when virtual machines are owned by mutually  
untrusting users. Secondly, it is necessary to support a variety of  
different operating systems to accommodate the heterogeneity of popular  
applications. Thirdly, the performance overhead introduced by  
virtualization should be small.  
  
Xen uses a technique called paravirtualization: The guest OS is  
modified, mainly to enhance performance.  
  
The Xen hypervisor (microkernel) does not provide device drivers for  
your hardware (except for CPU and memory). This job is left to the  
kernel that's running in domain 0. Thus the domain 0 kernel is  
privileged; it has full hardware access. It's started immediately after  
Xen starts up. Other domains have no access to the hardware; instead  
they use virtual interfaces that are provided by Xen (with the help of  
the domain 0 kernel).  
  
Xen does support booting other Operating Systems; ports of NetBSD  
(Christian Limpach), FreeBSD (Kip Macy), and Plan 9 (Ron Minnich)  
exist. A port of Windows XP was developed for an earlier version of  
Xen, but is not available for release due to license restrictions.  
  
In addition to this package you need to install the kernel-xen and  
xen-tools to use Xen. Xen 3 also supports running unmodified guests  
using full virtualization, if appropriate hardware is present. Install  
xen-tools-ioemu if you want to use this.  
  
[Hypervisor is a trademark of IBM]