Ubuntu vs. Kubuntu vs. Xubuntu [closed]

I don't have a link handy but the main differences are the following:

Ubuntu:

• Main distribution
• Uses Unity as the desktop environment
• Close to what people sees when people says "Linux desktop"

Kubuntu:

• Uses KDE as the desktop environment
• Pretty-looking. I personally describe it as a Mac OS X/Windows hybrid, visually speaking.
• Generally slower on lower end/older hardware
• Might use more disk space.
• Has the best MIME-Type association system/launcher, in my humble opinion. (kde-open)

Xubuntu:

• Uses XFCE as the desktop environment
• XFCE is a fork of the old GNOME desktop, resembles GNOME 2 in appearance, aimed at being lightweight but still usable.
• runs decently on lower end/old hardware
• Not exactly "super fast", but better faster than GNOME/Unity/KDE

There's also Lubuntu and Fluxbuntu which are using different desktop environments, respectively LXDE and Fluxbox. Fluxbox would be the fastest and lightest, but not exactly user-friendly, especially if you come from another non-unix-line Operating System. Other distributions that are lighter and that might work better depending on how old and powerful this computer is, but I will let you do the research about those.

As noted in this answer's comments, LXDE is not actively developed anymore.