I recently installed Windows Server 2008 with Hypervisor. I was a little confused by the Snapshot options. But after reading this post, and a little trial and error, I figured it out…
Apply… results in losing your changes since the snapshot. This applies the state stored in the snapshot to the current state of the machine.
Delete Snapshot… results in keeping your changes since the snapshot. This causes the AVHD (differencing disk) files to be merged into the parent VHD or AVHD files. Note that if the virtual machine is running the merge will not happen until the guest machine is shut down.
Revert does the same thing as Apply for the most recent snapshot.
Additional Notes from my experience: When you create multiple snapshots you’ll see that Hypervisor creates a hierarchy of snapshot nodes. This hierarchy is important. The green arrow is also important. I made the mistake at first of thinking that the green arrow and the snapshot were the same thing, but the green arrow represents the current state and all changes that have happened since its parent snapshot. Once I understood this, it was much more clear to me why apply resulted in losing my current changes and delete resulted in keeping my current changes