Quoting ArsTechnica
The latest release of the Linux kernel, 2.6.20, includes integrated virtualization capabilities with the Kernel-based Virtual Machine (KVM). The KVM kernel module, the development of which was largely sponsored by start-up tech company Qumranet, leverages x86 virtualization extensions included in various Intel and AMD processors. Several distributions, including Ubuntu and Fedora, are already preparing to include the KVM kernel module in upcoming releases.
KVM provides support for "full" virtualization, which means that, like VMware, it uses simulated hardware to allow users to run unmodified versions of other operating systems simultaneously. It can take advantage of processors that support either Intel's VT extensions or AMD's SVM extensions, and it uses a modified version of Qemu in userspace. Like the Xen virtualization system, KVM also has paravirtualization support, but it is still highly experimental. Modern paravirtualization technologies enable modified guest operating systems to direct system calls to a hypervisor rather than emitting machine instructions that are interpreted by a virtualized hardware layer. The recent addition of paravirtualization support to KVM, which uses an ad-hoc hypercall API, has improved performance by 30 percent in some contexts.
Since paravirtualization only works with guest operating systems modified to interact with a hypervisor, proprietary operating systems traditionally haven't been supported by open source paravirtualization technologies. A technical collaboration roadmap recently released by Microsoft and Novell reveals that the two companies are collaborating to bring support for Xen's hypervisor to Microsoft's upcoming Windows Server release, meaning that Xen will eventually be able to run Windows Server as a guest operating system with full paravirtualization capabilities. The KVM developers plan to use the same paravirtualization interfaces already used by Xen and VMware. Such compatibility could eventually make it possible for Windows Server to run paravirtualized in KVM.
Since KVM is now officially supported by the Linux kernel, the module will be available in some upcoming Linux distribution releases. Red Hat Enterprise Linux (RHEL) Product Management Director Joel Berman has stated that KVM is being packaged for Fedora, but Red Hat will not include KVM in the upcoming release of Red Hat's commercial distribution for enterprises, which already includes Xen. Ubuntu developers have also built a KVM package for Ubuntu 7.04, the upcoming Ubuntu release.
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