It is possible, to install KDE run
sudo apt-get install --no-install-recommends kubuntu-desktop.
After you install it you can switch between Desktop Environments at the login screen.
This also works with most of the common DE's out there.
For Xubuntu (XFCE)
sudo apt-get install --no-install-recommends xubuntu-desktop
For Lubuntu (LXDE)
sudo apt-get install --no-install-recommends lubuntu-desktop
And Gnome Shell (Gnome 3)
sudo apt-get install gnome-core
Theoretically you can have as many of these installed as you want, but I would only use one or two at time to avoid conflicts.
Note: If you plan on moving to a new DE completely, remove the
--no-install-recommends from the commands above.
You can remove any of these with the
purge command in
apt, like this:
sudo apt-get purge gnome-core
sudo apt-get purge kubuntu-desktop
If you want to try Linux Mint's Cinnamon use these commands:
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable
sudo apt-get update
sudo apt-get install cinnamon
To uninstall Cinnamon use
sudo apt-get purge cinnamon and follow these instructions on removeing PPA's.
I have personally run Unity and Kubuntu, Unity and Xubuntu and Unity and Gnome Shell. I have used Cinnamon a little bit too. KDE is by far the most customizable closely followed by XFCE in Xubuntu. Right now I just use Unity.