When creating a Highly Available Hyper-V environment, a Failover cluster needs to be created. A Failover cluster is a logical server consisting of two or more Windows Server 2008 servers. Server 2008 supports a maximum of 16 servers in a Failover cluster. These servers’re called Cluster Nodes. All Cluster Nodes in Failover cluster’re connected to a shared storage solution where the data is stored. VMs running under Hyper-V that need to be highly available’re configured as a resource in cluster. Note. All servers in the cluster must run the same Operating System, all nodes must either be Enterprise or Datacenter Edition and for Live Migration all servers must run the same processor family (even the same processor stepping). In a Server 2008 Failover cluster, only one Cluster Node can be “owner” of the disk resource and only the owner of the disk resource can access the files (including the Hyper-V VHD files). Therefore, if a VM needs to be failed-over to another Cluster Node, the complete disk resource needs to be brought down, moved to the other Cluster Node and be brought online again.
Full Article: Hyper-V R2 Live Migration