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Xen 3.0 Virtualization Interface Guide
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1. Introduction

Xen allows the hardware resources of a machine to be virtualized and dynamically partitioned, allowing multiple different guest operating system images to be run simultaneously. Virtualizing the machine in this manner provides considerable flexibility, for example allowing different users to choose their preferred operating system (e.g., Linux, NetBSD, or a custom operating system). Furthermore, Xen provides secure partitioning between virtual machines (known as domains in Xen terminology), and enables better resource accounting and QoS isolation than can be achieved with a conventional operating system.

Xen essentially takes a `whole machine' virtualization approach as pioneered by IBM VM/370. However, unlike VM/370 or more recent efforts such as VMware and Virtual PC, Xen does not attempt to completely virtualize the underlying hardware. Instead parts of the hosted guest operating systems are modified to work with the VMM; the operating system is effectively ported to a new target architecture, typically requiring changes in just the machine-dependent code. The user-level API is unchanged, and so existing binaries and operating system distributions work without modification.

In addition to exporting virtualized instances of CPU, memory, network and block devices, Xen exposes a control interface to manage how these resources are shared between the running domains. Access to the control interface is restricted: it may only be used by one specially-privileged VM, known as domain 0. This domain is a required part of any Xen-based server and runs the application software that manages the control-plane aspects of the platform. Running the control software in domain 0, distinct from the hypervisor itself, allows the Xen framework to separate the notions of mechanism and policy within the system.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire