COLO FT/HA (COarse-grain LOck-stepping Virtual Machines for Non-stop Service)
project is a high availability solution. Both primary VM (PVM) and secondary VM
(SVM) run in parallel. They receive the same request from client, and generate
response in parallel too. If the response packets from PVM and SVM are
identical, they are released immediately. Otherwise, a VM checkpoint (on demand)
is conducted.

See the website at http://wiki.xen.org/wiki/COLO_-_Coarse_Grain_Lock_Stepping
for details.