Microsoft - Live Migration included free in Windows Server

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 (there is no extra charge, that is, it's included with every Windows Server licence). Since Hyper-V live migration can move running virtual machines without downtime, it facilitates greater flexibility and value.