I'm not a Hyper-V guy but I'm sure the concept is the same as VMware practices. Virtual DC backups still have to be performed via an in-guest agent that captures the system state. Snap based backups aren't supported and I could only assume that has to carry over to Hyper-V.
As everyone else has said, you'll have to do a backup of the System State from within the VM to capture the data. This can be agent-based (whatever program) or generic windows Server Backup, etc, but from within the VM. After that Data is captured, full VM image level is fine.