On attempting to run your Virtual Machines on OS X 10.9 Mavericks these messages may appear:
Your virtual machine may run slow because virtualization support is disabled on your Mac
The virtual machine performance may be degraded because the hardware virtualization support is disabled on the host computer. Unable to load the Parallels Driver
Parallels Desktop for Mac kernel extensions are not loaded on OS X Mavericks.
Uninstall Parallels Desktop
Open Finder, go to Applications folder -> Utilities; Launch Disk Utility
Click on your Mac Hard Disk and press Repair permissions. Wait till the operation is done.
Reboot your Mac
Download Parallels Desktop installation package:
Install Parallels Desktop
Note: After you reinstall Parallels Desktop it is necessary to reboot your Mac to install it properly.
For installing OS X 10.9 Mavericks (release) in a Virtual Machine in Parallels Desktop 9 please check KB 118806
For installing OS X 10.9 Mavericks Developer Preview in a Virtual Machine please check KB 116282