There is Network Migration, Quick Migration, SAN Migration, and soon - Live Migration.
Most people get confused when they start talking about infrastructure and what is required for each to work. Then someone mentions the VDS Hardware Provider and Windows Storage Server, and the discussion usually goes to 'Why do I require Storage Server? I don't get it.'
My quick and dirty response is:
The hitch is SAN Migration. Quick / Live Migration is easy - Failover Clustering does that.
SAN Migration requires that you have a SAN and SAN management software that hooks into VDS (the Virtual Disk Service).
You install the SAN management agent on the SCVMM server.
It is here where most folks being talking about Storage Server - as it has a VDS Hardware Provider (SAN management agent) that is VDS capable.
Like I mentioned - Quick / Live Migration is easy - it is built in, it is SAN Migration that requires infrastructure.
Now, let me get into the greater details of each type of migration and who is performing it. (yes, these are marketing terms)
Network Migration and SAN Migration are specific to SCVMM.
Network Migration is the act of copying a VM over the wire, using BITS between two points (either between the Library and a Host, or a Host and a Host).
SAN Migration is the act of moving a VM between two points by detaching and reattaching a LUN (Library / Host or Host / Host)
The requirements for SAN Migration are: One VM per LUN, the SAN can be managed by SCVMM through VDS Hardware Provider software installed on the SCVMM server, all entities can talk to the SAN. SAN Migration generally involves Fiber Channel SAN connections.
Quick Migration and Live Migration are specific to Hyper-V.
Both use Failover Clustering to move a VM between two Hosts. (The SCVMM Library is not involved at all). All the requirements of Failover Clustering apply (shared storage, similar config, similar hardware, etc.).
In both cases the VM must be manged by Failover Clustering (which is included in all flavors of Hyper-V), also referred to as making the VM Highly Available.
Quick Migration is available with the v1 of Hyper-V. When a Quick Migration is triggered, Failover Clustering saves the VM, moves the ownership to the failover host, then starts the VM on the new host.
Live Migration will be available in the R2 release of Hyper-V. It is very similar to Quick Migration except that the VM is not saved - it is running during the entire operation. I am not going to go into the details in this post.
I hope that helps a few folks with clearing up the confusion between the terms, the high level technicals, and the infrastructure that you might need.