Ivan Pepelnjak explains the difference between hot and cold VM mobility:

Another day, another interesting  Expert Express  engagement, another stretched layer-2 design solving the usual requirement: “We need inter-DC VM mobility.”

The usual question: “And why would you want to vMotion a VM between data centers?” with a refreshing answer: “Oh, no, that would not work for us.”

Read the complete story at  http://blog.ioshints.info/2013/02/hot-and-cold-vm-mobility.html