I know that it stands for Exposure Value, so my 6D focuses in 2.5EV worse light than a 600D.
But it's relative to... what?
What makes it meaningful? What's the "base unit" for AF sensitivity?
Exposure Value is actually an absolute scale, where EV 0 corresponds to an nominally-exposed photo having an exposure time of 1 second at aperture f/1.0 (or of course equivalents, like 2 seconds at f/1.4).
You can see by that baseline that -3EV is pretty amazing — it corresponds to a minute-long exposure at f/2.8.
We're used to seeing EV in the context of EV compensation, where the camera is told to expose differently from what its meter tells it to, and that's expressed in relative terms (plus or minus some number EV), but the basic scale is actually fixed. More on this a What is the EV scale? and What is exposure compensation?