We have Postscript coming and going. In my case right now, I have a postscript file that I would like to send to my Brother printer (which only knows about PCL6). At one time many people (myself included) had postscript printers (my old HP laserjet) that expected postscript rather than PCL.
PCL stands for printer control language. Many printers these days implement this (as does my current Brother printer). PCL was invented by HP.
XPS is a microsoft invention. It stands for "XML Paper Specification". It is essentially an alternative to PDF that I never heard about until I started digging into all of this. Note that PDF and Postscript are brothers of sorts. Both were invented by Adobe and there is some common ground.
And then we have Unidrv. This was a Microsoft invention that is now deprecated, with the intent that XPS would take its place. It was intended to be an alternative to postscript. It is not clear that Unidrv or XPS have ever been adopted by printer manufacturers, which was probably Microsofts intent.
If you look at the Ghostview/Ghostscript page, you will find the following offered:Adventures in Computing / tom@mmto.org