Live Migration is a great solution for planned downtime such as servicing the underlying hardware like adding more memory, storage or applying a BIOS update. Simply Live Migrate the virtual machines to another server (without downtime) shutdown the physical server and perform the maintenance. When the maintenance is complete, Live Migrate the virtual machines back and your done. High Availability is a great solution for unplanned downtime. For example, suppose someone accidentally unplugs the wrong power cable on a server. The virtual machine on the server that just unexpectedly went down will automatically restart on another node without any user intervention.
More info: WVT blog