Different Desktop Managers of Ubuntu

First of all, X Window Core must be installed, or you won’t be able to use GUI. X Window Core is called “x-window-system-core”. To install any GUI, you should install this first.

In Linux, one of the display managers (login managers) gdm/kdm/xdm should be chosen. To get a different desktop manager for ubuntu, just add packages following:

gnome: ubuntu-desktop
kde: kubuntu-desktop
xfce: xubuntu-desktop

e.g.

(root)# apt install ubuntu-desktop

After the installation, of course, we should set default-display-manager on ubuntu. There are two ways to do so,

Edit file “/etc/X11/default-display-manager”, change display manager name in it and save, then reboot.
Run “dpkg-reconfigure gdm/kdm/xdm” to switch (gdm for gnome, kdm for kde, xdm for xfce), you will get a menu to choose. Of course you should do this as root, or “sudo”.

Refer to: HOWTO: correctly install/remove k/x/ed/ubuntu-desktop packages