Hyper-V Quick Migration is a feature that allows users to move running virtual machines (VMs) from one host to another with minimal downtime. Live Migration replaced Quick Migration in Windows Server 2008 Hyper-V R2 and has been part of every subsequent release.
The amount of time it takes to start a virtual machine on another host with Quick Migration depends on the amount of RAM in the virtual machine and the connection speed to the storage. With Quick Migration, a VM is put in a saved state; the VM's memory information is transferred to another host which then starts the VM. Quick Migration requires shared storage and Windows Server Cluster, which is only available in Windows Server 2008 Enterprise and Datacenter. A failover cluster is required for both planned and unplanned downtime in every version up to Windows Server 2008 R2; without one, there will be no safety net in the event of a server failure.
In Windows Server 2012 Hyper-V, Microsoft also introduced shared-nothing Live Migration, which only requires network connectivity and assists with planned VM downtime.