By default Ubuntu desktop installation will install gnome desktop enviroment and if you want to install KDE desktop enviroment you have three options.Those options are
kubuntu-desktop -This is the recommended metapackage to install; the full Kubuntu installation, with all the Kubuntu recommended packages. This includes OpenOffice, Kontact, Konversation, amaroK, K3B, and others.
kde -This will install the following KDE packages: kde-amusements, kdeaccessibility, kdeaddons, kdeadmin, kdeartwork, kdegraphics, kdemultimedia, kdenetwork, kdepim, kdesdk, kdeutils, kdewebdev, kdevelop3 and the kde-core metapackage
kde-core -This will install the core --- the bare-minimum required-- of KDE. That is, kdebase, kdelibs, arts and fontconfig.
If you choose to not install kubuntu-desktop, then you can still get all the Kubuntu-specific tweaks by installing the kubuntu-default-settings package.
First thing you need to make sure you have universe source list under /etc/apt/sources.list file
If you want to install kubuntu-desktop use the following command
sudo apt-get install kubuntu-desktop
This will starts installing all the required packages
During this installation process, you will be asked whether you want to use KDM or GDM as your default display manager. If you think you'll use KDE , make KDM your default. If you think you'll use Gnome , keep GDM as your default.Select which one is best for you.
This will complete the installation now you need to logout from ubuntu gnome session and now you are on ubuntu login screen in this you need to select options---> selectsession
You should see the following screen with gnome and kde options you have select kde and click on change session
Now it will disply the following screen in this you want to change to KDE just for this session or if you want to make KDE your default desktop environment.
Once you select the above option it will start loading KDE session you can see this in the following screen
Once it loads complete KDE session here is the KDE desktop for your ubuntu.
Nice KDE Desktop
If you want to switch back to Gnome, just log out and select Gnome from the session menu.
If you want to install only KDE use the following command
sudo apt-get install kde
If you want to install only kde-core use the following command
sudo apt-get install kde-core
Uninstall KDE in ubuntu
If you want to uninstall KDE session use the following command
sudo apt-get remove kubuntu-desktop