Server virtualisation is all about decoupling the operating system form the hardware it’s running on, and one of the benefits of doing this is to ensure that a virtual machine (VM) can be made resilient to any underlying hardware failure. In the world of Hyper-V this is achieved by building a Windows Server cluster and adding the VM as a role into that cluster. From Windows Server 208 R2 this also gives the benefit of moving the virtual machine around nodes on the cluster without stopping the virtual machine (known as live migration).
In Windows Server 2012 you still need to use a cluster to make virtual machines highly available, but you also have the option to build a cluster without any shared storage using a file share to host the virtual machines storage and metadata. This screencast shows how that works..