If you like to use KDE you probably install Kubuntu directly, instead of Ubuntu, which has been based on Gnome for a long time now.
However, I like to have several Desktop Environments, and, now and then, I like to switch from Gnome to KDE and then back. Currently, I’m using Gnome for most of the time, that’s why I install Ubuntu (instead of Kubuntu).
In any case, you can still install KDE Plasma on top of Ubuntu. The following has been tested on an Ubuntu Disco 19.04, but I guess it will work also on previous distributions.
For a reduced installation of KDE you might want to install only these packages
sudo apt install kde-plasma-desktop kde-standard kwin-addons
In particular, kwin-addons includes some useful things: it contains additional KWin desktop and window switchers shipped in the Plasma 5 addons module.
When installation has finished you may want to reboot and then, on the Login screen, you can use the gear icon for specifying that you want to enter the KDE Plasma environment instead of the default Gnome environment.
The above packages should provide you with enough stuff to enjoy a Plasma experience, but it lacks many (K)ubuntu configurations and addons for KDE.
If you want more Kubuntu stuff, you might want to install the “huge” package:
sudo apt install kubuntu-desktop
And then you get a real Kubuntu KDE Plasma experience.
Note that this will replace the classic Ubuntu splash screen when booting the OS: it replaces it with the Kubuntu splash screen. If you want to go back to the original splash screen it’s just a matter of removing the following packages:
sudo apt remove plymouth-theme-kubuntu-logo plymouth-theme-kubuntu-text
Remember that you can also use the Kubuntu Backport PPA for enjoying more recent versions of KDE software.
Enjoy Gnome and KDE! 🙂