I found myself in a situation where I had a 1 TB drive on a VM host (running Windows 2008 R2) with four virtual machines on it that was completely out of space. I found that there were tons of AVHD files taking up a lot of room. Per what I read, these are snapshot files, even though I deleted the snap shots from the Hyper-V Manager (and cleared up a dozen GB or so), the snapshots were not truly gone.
This page talks a little bit about the process:
But in the end, I got the space back by just turning off the virtual machines. When I did so, there was a merge process that went on with the snapshots. In a turned off machine, this just happens automatically. In the end, I turned off the machines and left them off until the "cancel merge in progress" went away. For my largest VM (approx 400 GB of storage), this took about 4 hours.
This is what the cancel merge in progress looks like: