I've checked the packages that apt-get install would install for each of these. These aren't complete lists, but only list the differences from my Unity (ie., without any KDE programs) setup.
plasma-netbook and plasma-desktop differ only by whether they install the netbook or the desktop interface. kde-workspace is identical to plasma-desktop.
kde-plasma-desktop will install a few KDE programs: Dolphin, Konqueror, kde-baseapps, konsole, kwrite, and kde-passwd.
kde-standard installs a bunch of stuff on top of kde-desktop: Akregator, Ark, Dragonplayer, gnupg, Kaddressbook, Kate, KDE wallpapers, Kmail, etc.
kubuntu-desktop installs even more than kde-standard. kubuntu-desktop is the package you would download from the Ubuntu website if you wanted Kubuntu, but if you only want the Plasma interface, not any of the default programs, plasma-desktop might be sufficient.
Let me try to clarify a bit for you.
KDE plasma is about the same as gnome-shell, it is the most recent iteration of KDE.
You can have multiple DE installed, in this case both gnome and kde, without a problem and you can choose which one to log into at the login window .
Right now, KDE can not run on wayland, and is in the process of transitioning to wayland.
gnome-shell can run on either wayland and X11 .
"gnome" is a desktop environment, it includes a bunch of packages other than the window manager such as gedit, nautilus, a number of servies, etc.
gnome-shell is the window manager.
kde is a desktop environment and includes a bunch of packager such as kate, k3b, a number of kde services, etc.
plasma is the window manager (well kwin is the window manager and plasma is the window manager + desktop features such as widgets but generally "plasma" does not generally refer to the additional packages such as kate, k3b, etc).
see What is the difference between a desktop environment and a window manager?
so gnome and kde include a large number of packages of which gnome-shell and kde plasma (kwin) are a part.
Best Answer
Running
apt showon each:kubuntu-desktop
kde-plasma-desktop
plasma-desktop
Look at the depends for each in the output of each
apt showcommand 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.