There’s two ways…through the parent and then blind to the parent.
Hyper-V Backup Strategies:
A good discussion there.
Blind to the parent way:
Take an external usb hard drive or nas and in the HyperV server, build a virtual drive of sufficient size. Make it a dynamic, not fixed drive size. Now add that dynamic disk/vhd as a drive to your hyperV children. Once you’ve done that you can use the vhd as a normal drive and any Windows backup will see it as just another hard drive.
Through the parent way:
This will only work with Windows 2008/ 2008 r2 or 2012 – not with Windows 8. http://blogs.technet.com/b/askcore/archive/2008/08/20/how-to-enable-windows-server-backup-support-for-the-hyper-v-vss-writer.aspx and http://support.microsoft.com/kb/958662older info on the topic - http://www.windowsitpro.com/article/virtualization/q-how-can-i-check-if-the-backup-integration-service-is-running-in-my-guest-os-in-hyper-v- how backup assist does it http://www.backupassist.com/blog/support/hyper-v-backup-using-windows-server-backup/