If you have a running Hyper-V Virtual Machine with is not part of a cluster you can make this virtual machine highly available via the Failover Cluster Manager. First of all the Virtual Machine has to be stored on a shared storage. In Windows Server 2008 R2 this means the Virtual Machine and the attached Virtual Disks had to be stored on a Cluster Shared Volume (C:\ClusterStorage\…), with Windows Server 2012 the VM could also be stored on a SMB 3.0 file share.
And there is also a big difference in Windows Server 2008 R2 Hyper-V and in Windows Server 2012 Hyper-V. In Windows Server 2008 R2 the Virtual Machine had to be offline during you add the VM to the Cluster, and in Windows Server 2012 you can do this online.
First open the Failover Cluster Manager and right click on your Cluster Name in the menu select “Configure Role”
This started the High Availability Wizard. First select the role you want to make high available, in the Hyper-V case this is “Virtual Machine”.
Now select the Virtual Machine you want to add to the cluster. Note: In Windows Server 2012 the Virtual Machine can be online, in Windows Server 2008 R2 the Virtual Machine had to be offline.
Btw. thanks to Philipp Witschi (itnetx), who I now owe a dinner.