Xen 是一个开放源代码虚拟机监视器，由">剑桥大学开发。它打算在单个计算机上运行多达100个满特征的操作系统。操作系统必须进行显式地修改（“移植”）以在Xen上运行（但是提供对用户应用的兼容性）。这使得Xen无需特殊硬件支持，就能达到高性能的虚拟化。
Xen通过一种叫做准虚拟化的技术获得高性能，甚至在某些与传统虚拟技术极度不友好的架构上（x86），Xen也有上佳的表现。与那些传统通过软件模拟实现硬件的虚拟机不同，在Intel VT-X支持下3.0版本之前的Xen需要系统的来宾权限，用来和Xen API进行连接。到目前为止，这种技术已经可以运用在NetBSD, GNU/Linux, FreeBSD和Plan 9系统上。在Brainshare 2005会议上，Novell展示了NetWare与 Xen的连通。与Windows XP连通的技术曾在Xen开发初期进行，但微软的协议未能允许它发布。Sun微系统公司也正在积极地将Solaris移植到Xen平台之上。
Xen 4.1更新如下：
·A re-architected and improved XL toolstack that is functionally nearly equivalent to XM/XEND.
·Prototype credit2 scheduler designed for latency-sensitive workloads and very large systems.
·CPU Pools for advanced partitioning.
·Support for large systems (>255 processors and 1GB/2MB super page support).
·Support for x86 Advanced Vector eXtension (AVX).
·New Memory Access API enabling integration of 3rd party security solutions into Xen virtualized environments.
·Optimizations for Linux HVM guest IRQ delivery when running PV-on-HVM drivers.
·Fixes to xenpaging and memory sharing, but they are still considered as 'Tech Previews'.
·Tmem fixes, but still disabled as a default (enable with hypervisor cmdline option).
·Remus FT (Fault Tolerance) fixes.
·Documentation additions (vbd-interface.txt, vbd numbering and naming, xl disk configuration syntax, etc).
·Fixes to properly support jumbo frames (mtu 9000) with vif-bridge script.
·Many IOMMU fixes (both Intel VT-d IOMMU and AMD IOMMU).
·Many toolstack and buildsystem fixes for Linux and NetBSD hosts.
·Thirdparty libs: libvirt driver for libxl has been merged to upstream libvirt.
·HVM guest PXE boot enhancements, replacing gPXE with iPXE.
·Interrupt (IRQ) delivery fixes, fixing keyboard/mouse on some laptops.
·Xentrace and xenoprofile fixes for analysing the hypervisor and VMs.
·Userspace qemu-based block device backend driver to use when dom0 kernel does not have kernel based xen-blkback driver ·available (upstream Linux 2.6.39 does not yet have it, for example).
·Even better stability through our new automated regression tests.
下载地址：
Xen Hypervisor 4.1: Download (archives) Xen Cloud
Platform1.0: Download (archives) Xen Hypervisor 4.0.1: Download (archives)