hvm tap vs vif
PV虚机只有一个vifX.N，而HVM虚机不安装PV Driver的时候会有两个网络接口vifX.N和tapX.N，而且实际上使用的是tapX.N。
HVM虚机安装PV Driver并重启后，tapX.N会消失，使用vifX.N（若不重启还是使用tapX.N）
tap interfaces -> qemu-emulated NICs (realtek model by default), used
when no PV drivers installed (using OS' own driver for that model)
vif interfaces (the one that you can rename) -> xen network frontend
NICs, used when you have PV drivers installed (like GPLPV,
http://meadowcourt.org/downloads/)
when no PV drivers installed (using OS' own driver for that model)
vif interfaces (the one that you can rename) -> xen network frontend
NICs, used when you have PV drivers installed (like GPLPV,
http://meadowcourt.org/downloads/)