You are partly right, you can just copy the VHD's to get backups of a virtual hard disk, however the virtual machine will likely have to be shutdown unless you use an application which has an agent for Hyper-V to enable online backups.
There's a good technet article here on backup strategies for Hyper-V;
http://technet.microsoft.com/en-us/library/dd252619(v=ws.10).aspx
The important part of the article is probably this;
Understanding online and offline backups
Whether a backup is performed online or offline depends on whether the backup can be performed without downtime. You can perform an online backup with no downtime on a running virtual machine when all of the following conditions are met: Integration services are installed and the backup integration service has not been disabled.
All disks being used by the virtual machine are configured within the guest operating system as NTFS-formatted basic disks. Virtual machines that use storage on which the physical partitions have been formatted as dynamic disks or the FAT32 file system prevent an online backup from being performed. This is not the same as dynamically expanding virtual hard disks, which are fully supported by backup and restore operations.
Volume Shadow Copy Service must be enabled on all volumes used by the virtual machine with a specific configuration. Each volume must also serve as the storage location for shadow copies of the volume. For example, the shadow copy storage for volume C: must be located on C:.
If an online backup cannot be performed, then an offline backup is taken. This type of backup results in some degree of downtime. A variety of factors can affect the time required to take an offline backup. If the virtual machine is running or paused, it is put into a saved state as part of the offline backup process. After the backup is completed, the virtual machine is returned to its existing state.