I don't think XPV is documented anywhere.. but it is the concept of using the math of XP to determine level and vice versa.
If you take a Solo monster and level it up by 4, its XP value is equivalent to an Elite. Then again from elite to Standard, and then again from Standard to Elite.
Now if you take 8 minions and add their XP value, it comes out to be the a Standard monster of 4 levels higher...
Examples:
Savage Orc brute minion-4
In an 8 man squad they become an 8th level Swarm standard monster, size 2x2.
In a 128 man company they become a 24th level standard monster, size 8x8.
Human Town Guard soldier standard-3
In an 8 man squad they become an 11th level Swarm elite monster, size 2x2.
In a 128 man company they become a 27th level Swarm elite monster, size 8x8
etc...