The new Hyper-V R2 will ship with Live migration (LM), the most requested Hyper-V feature.
What it can do is moves a Virtual Machine from one node of the Windows Server Failover Cluster (WSFC) to another node in the same cluster in a fashion that is transparent to the clients that are connected to it. This is done with no dropped network connections, no loss of service, no perceived user downtime.
Live Migration is supposed to only take the VM offline for less than 1 second, compared to the current Quick Migration which will take a few seconds.
Live migration is accomplished by replicating and synchronizing the contents of memory and processor(s) being used by the VM and then quickly switching access to the storage and network from the source node to the destination node. This switch is done quickly enough that the network sessions from client systems connecting to the VM are not lost and therefore has little to no impact on the service that the VM is providing.
I am eager to test it out!!!