In earlier versions of Windows Server you needed to build a cluster with shared storage (i.e. a SAN) if you wanted to move a virtual machine from server to server without stopping it (known as Live migration in Hyper-V). In server 2012 you just need to configure Live Migration in each of the servers as per this screencast..
But why does this matter? in a word - agility. Particularly for smaller businesses who don't have the budget or expertise to run a SAN, and for whatever reason want to manage their services in house rather than use the cloud. Key services can be moved around as needed without stopping them and this means that planned maintenance tasks can be carried out during the working day.
Setting this up is really easy and we usually get our delegates at our IT Camps to pair up and do this using their own laptops without too many problems. If you have two desktops/laptops lying around you can get and an evaluation copy of Windows Server 2012, and follow along.
Things to note.
The number of live migrations you configure is up to but if you only have limited networking you’ll set this low as you don’t want to interfere with access to the VM if that traffic is on the same network (You can set live migrations to use specific IP addresses).
You can use CredSSP or Kerberos (i.e. the host machines are in the same domain) to setup the trusts between the hosts for this to work. Note the domain etc. of the virtual machine isn’t relevant
There is no high availability here – If the host running the VM stops working so does the VM and if the host suffers a disk crash the virtual machine will be gone as well, so this technique just helps with planned maintenance.