How can I have ubuntu and xubuntu as different os, so that when I turn on my computer I can choose between ubuntu and xubuntu or other ubuntu variant?
It depends on things that you want the two distinct OSes to have in common.
If you want them to have nothing in common, just make two partitions with one flavour of Ubuntu in each partition.
If you want them to have some things in common (typically your personal datas) make partitions where you will mount that shared things in common (
PS : why not installing one Ubuntu OS and then install different desktops environments on it (Gnome (already installed with Ubuntu), XFCE--with the xubuntu-desktop package), KDE with the kubuntu-desktop package if you want to have Kubuntu installed...) ? You can select which one you want when you login.
To install Xubuntu alongside Ubuntu, just install a clean install of one OS (erase whole disk), then install the other using the "Install *Ubuntu alongside existing OS" option in the liveCD installer. The GRUB bootmenu should have an option for either OS when you first boot up.