One of two things is going on. S&W may have originally named the 5" barrel version differently than they do now and I am unaware of it. However, I have never known the 460V to be anything other than the 460V, and I bought mine when they were originally in production back in 2009. They were dropped for awhile, and then brought back about a year or two ago. Though, the 460 cartridge was around a few years before I got into it, so the naming may have been changed.
I think the more likely thing that is going on is that the people selling these guns have them mislabeled. It's a simple mistake and I've seen it on Gunbroker plenty of times.
XVR does stand for Extreme Velocity Revolver.
460V is as I stated it, just a Roman numeral reference to the barrel length. They probably wanted to avoid the confusion of calling it the 4605 or 460-5, so they went with V.
You could check on the S&W forum if you don't want to take my word for it.
Your question about performance differences is like I said, the original 460 gun was designed with a longer barrel, so listed velocities and trajectories you see on 460 ammo will be a bit higher than you will get out of a 5" barreled gun. Don't let the difference in designation bother you though. You still get a lot of speed out of the 5" barrel. I plan to buy a chronograph hopefully sooner than later, and will post some speeds when I have them. I picked up a Ruger No1 in 460 mag last summer, and I want to see what kind of speed I'm getting out of a 22" barreled rifle with this cartridge.
They could have called it the HVR for "High Velocity Revolver", or the 460SSTTXVRBSRPF which of course stands for "Somewhat Slower than the XVR but Still Really Pretty Fast".
Of course if we're going to go there, the 460's with 12 inch barrels could be called 460TNR for "Tactical Nuke Revolver.
