What's the difference between
1.installing Ubuntu and changing its desktop to KDE or direct installing kubuntu
2.installing Ubuntu and changing its desktop to XFCE or direct installing xubuntu
3.installing Ubuntu and changing its desktop to LXDE or direct installing lubuntu
4.installing Ubuntu and changing its desktop to mate or direct installing Ubuntu mate
5.installing Ubuntu and changing its desktop to gnome or direct installing gubuntu
Please help... I am confused..
They're all Ubuntu-based distros with different tweaks. Each distro runs on the same Linux kernel, so they're the really same for most intents and purposes. The obvious difference, as you have stated, is the desktop manager installed on each one, so a different look and feel. However, each of them also comes with different packages pre-installed. Each distro may ship with different video viewers, photo viewers, email clients, photo editors, and so on.
Each one will also consume different amounts of RAM, CPU, and power.
For more information, this post has very good tables and graphs to compare the distros: http://mylinuxexplore.blogspot.com/2014/11/ubuntu-1410-vs-kubuntu-1410-vs-xubuntu.html