I see they differ in size.
plasma-desktop
is a dependency of kde-plasma-desktop
and it seems that kubuntu-desktop
and kde-plasma-desktop
are independent of each other.
What are other differences between kubuntu-desktop, plasma-desktop and kde-plasma-desktop packages?
Running
apt show
on each:kubuntu-desktop
kde-plasma-desktop
plasma-desktop
Look at the depends for each in the output of each
apt show
command to know more.In brief, kubuntu-desktop provides a fuller KDE Plasma desktop experience relative to that obtained by installing kde-plasma-desktop.
It's a bit like installing xubuntu-desktop versus xfce4.
By the way, kubuntu-full provides even more packages than kubuntu-desktop.
I don't know the basis for the question, but I prefer not mixing desktop environments. Other users seem to handle multiple desktop environments just fine.