Alright, a bit of physics:
On a Per Photon basis, the higher the frequency, the greater the energy. This universally means greater damage. Also, X-Rays and Gamma Rays will penetrate materiels, but for something on the size range of a capital ship, the amount that would actually pass through would be negligeable. Also, X-Rays and above have a nasty secondary effect....when they strike solid materiels, their photons have enough energy to actually cause severe secondary radiation from what they hit. So anything hit by a gamma ray laser is now radioactive for a long time.
A Xaser is simply slang for X-Ray laser, so it fires coherent X-Ray light. Very mean, the only way we can accomplish this now is by the detonation of a nuclear bomb.
As for the Kayser, presumably the amount of ZP energy released is quite small, or it would be taking chunks out of Sathanas. Out of all the weapons, it's the most 'out there' as Zero Point Energy (or ZPE) is still poorly understood. We could build a crude X-Ray laser, I doubt we could build anything to take advantage of ZPE for quite a while. (BTW, Quantum torpedoes from ST use ZPE for their explosive power)
Finally, of course the maxim doesn't work like a proper railgun. None of the weapons save the beam cannons are anything like what real life energy (or not) weapons would be like.
