VirtualBox: Odd behaviour after updating the host to OS X Mavericks

Solution 1:

Reinstalling is your best bet.

VirtualBox installs drivers in your host kernel that enable it to function. When your kernel is changed -- such as by an OS upgrade -- those drivers are in the old one, but not in the new one. The easiest way to get them into your new kernel is to reinstall VirtualBox, which conveniently will also repair anything else that may have been "damaged" during the OS upgrade.

As @rxt mentioned in his comment, reinstalling VirtualBox won't lose any of your settings or VMs. It's the most expedient route to repairing your issues and getting you running again.