如下图：

Hypervisor： a Hypervisor is a very low-level,very lean and efficient(精简和高效) platform code layer that enables multiple operating system to run on host computer at the same time。

——是物理机器资源的基本抽象（I/O资源在VMware中是Hypervisor处理的，而在Xen中就不是）

——允许资源在虚拟机之间无破坏的分配（或重新分配）

virtuallization server：a virtuallization server includes a hypervisor function but also includes additional functionality：

——I/O resource/traffic management(Xen)

——server configuration

——access control

——security management