You don't actually need a separate partition. From your existing Ubuntu install:
sudo apt-get install kubuntu-desktop
sudo apt-get install xubuntu-desktop
Then log out and choose the new desktkop environment from the Sessions menu on the login screen. Couldn't be easier, and you don't have to mess with GRUB at all!
For testing non-Ubuntu distros without messing with GRUB, try Live CD, Live USB, or Virtualbox.
ps What does the FIRST LINE of your grub.cfg file say?