What is the correct pronunciation of Xubuntu?

The first one... Zoo-buntu!

That's how it's described here:

Xubuntu (Pronounced: /zù'búntú/) is an official Ubuntu-based operating system.

Lubuntu = Loo-buntu

Kubuntu = Koo-buntu, though I always seem to pronounce it as more of a Kuh sound.

Pragmatic answer: don't forget that these distros are used by people whose native language isn't English... any pronunciation which is not ambiguous/misleading for your audience is fine. Your audience should be able to infer the spelling from your pronunciation. Pronounce "Zoo-buntu" in front of French people and they will google for Zubuntu after the meeting...

Concerning Xubuntu, I go for ksoo-buntu...

My suggestion for Kubuntu would be Q-buntu. I pronounce Lubuntu as loo-buntu (with a short oo, unlike the toilet).