Ubuntu falls back to gnome after installing default because 3d-accellaration is deactivated without vm-additions. additionaly to running the installscript of the additions i had to: sudo apt-get update sudo apt-get install virtualbox-ose-guest-utils In Ubuntu 16.04 / 14.04 sudo apt-get install virtualbox-guest-x11 virtualbox-guest-dkms found here http://www.webupd8.org/2010/12/how-to-test-ubuntu-1104-with-unity-in.html