Hyper-V virtual machines can be backed up in a multitude of ways through the use of the backup integration service and the Hyper-V Volume Shadow Copy Service (VSS) Writer.

Microsoft recommends two methods for backing up virtual machines:

Perform Backup from the Server Running Hyper-V
This is the recommended form of backup. In order to do this, the backup software you are using must support the Hyper-V VSS Writer. In this method, you can perform a full server backup that helps backup all of the data required to fully restore the server. This includes everything from the configuration of the virtual machines and the virtual hard disks.

When performing a backup in this manner, you have the option of performing an online backup if certain conditions are met. This online backup means that the VM’s can be backed up without downtime. The conditions that must be met for online backups include:

Hyper-V Integration Services must be installed on the guest OS.
All disks used within the VM must be basic, and not dynamic disks.
Volume Shadow Copy Services must be enabled on all volumes used by the specified virtual machines. Each volume must also serve as the storage location for shadow copies of the volume. If an online backup is not supported, offline backups will be performed in the VM is put into a saved state while the backup is performed.

Perform Backup from within the Guest OS of a Virtual Machine
This backup option should be used in scenarios when the backup software you are using or the guest operating systems do not support the Hyper-V VSS Writer. In doing this, a backup application is run from the guest OS. This should be used in conjunction with a separate backup of the Hyper-V server itself.