Hyper-V Live Migration Overview & Architecture – Windows Server 2008 R2 & Microsoft Hyper-V Server 2008 R2

With Hyper-V live migration, you can move running VMs from one Hyper-V physical host to another without any disruption of service or perceived downtime. Live migration is integrated with Windows Server 2008 R2 Hyper-V and Microsoft Hyper-V Server 2008 R2. Since Hyper-V live migration can move running virtual machines without downtime, it will facilitate greater flexibility and value.

More info: Microsoft