摘自wikipedia: http://en.wikipedia.org/wiki/Hypervisor
1. 定义：a piece of computer software, firmware or hardware that creates and runs virtual machines.
别名：virtual machine manager (VMM)
2. 两类hypervisor（按照是否直接运行在硬件上来区分）
- Type I: 直接运行在硬件上，如Citrix XenServer, VMware ESX/ESXi
- Type II: 运行在操作系统上，如KVM
3. paravirtualization
Guest operating-system to make system calls to the hypervisor, rather than executing machine I/O instructions that the hypervisor simulates.
paravirtualization是Xen里的说法，其他产品对这个技术有不同的称谓：a "hypercall" in Parallels Workstation, and a "DIAGNOSE code" in IBM\'s VM.