Using Time Machine with Virtualization (Parallels, VirtualBox, VMWare, etc...)
Parallels has a specific setting to optimize its VMs for backup by Time Machine and prevent backing up unchanged data.


  To enable improved integration with Time Machine, please go to Virtual Machine Configuration, choose Options and hit Backup.
  
  Select "Optimize for Time Machine" in SmartGuard menu.


Similarly, VMWare Fusion has some support for Time Machine, which appears to involve turning on snapshots.