Xen and KVM

Xen :

Xen was first shipped in RHEL 5.0 and provided integrated virtualization capabilities for the first time. Paravirtualizatin techniques are used to allow modified operating systems to run on older x86 processors that do not have support to modern virtualization extensions. If the system processor does supports virtualization extensions, unmodified OS such as Windows Server 2003 can be run in a guest. The system is booted to run Xen hypervisor, which then boots a RHEL VM running a modified linux kernel. This first guest is privileged, and has ability to do control and access the resourses of other VM. The Xen hypervisor is and will continue to be fully supported RHEL on the 32bit x86, x86_64 and 64-bit ia64 version of the distribution.

KVM :

The kenrel based virtual machine integrates support for virtualization directly into the linux kernel. A normal, unmodified linux environment is booted as the host, which can then be used to start over virtual machines as guests. KVM needs processor vitualization support (i.e VT-Support), so it cannot be used on older x86 hardware. Operating system do not need to be modified to run on a KVM guest virtual machine. However KVM can use paravirtualization techniques to speed up virtual machine system performance if appropriate drivers for the VM OS are installed. In addition by integrating the hypervisor code into the kernel , the design of the system is actually simplified and support of the system is easier . KVM is fully supported in RHEL on the x86_64 arch starting with RHEL 5.4, and is the basis of the thin hypervisor product provided in RHE-V.