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
