When I logout on Xubuntu (xfce) I get a screen to login again, on top bar of this screen is placed an icon and when click it the drop down menu offers 3 options to login: Plasma, xfce and Xubuntu. By default the login is set to Xubuntu, but now I tried Plasma then I get KDE desktop screen and it stuck, displaying a error message that some packages/libraries are missing. I was able to open task manager and ended some processes to return to Xubuntu session. How to fix this issue? do I need to install some pcks to fix that? Another question : what the difference btw xfce and Xubuntu session? thnaks, vladi.
If you want to run KDE session you need install complete KDE desktop by using
sudo apt-get install kde-plasma-desktop.
I can assume you already have some KDE software in your installation which caused KDE session to appear in session chooser, but its not complete, valid installation so its obviously works with errors.
As fr "what the difference btw xfce and Xubuntu session" - there is not much of difference, just some preferences is different. Its just one is added by installing XFCE and another by installing "xubuntu-desktop" package, this is sometimes caused by installation of system from netboot install cd.