VU= volume units
FS= Full Scale
A VU meter isn't just a thing with a needle. It is a meter with particular balistics. (how fast the meter reacts to signal) The VU scale is always centered around line level. No mater whether line level is +4 or -10, the VU meters zero point will be that level
Peak meters are very fast and are scaled in negative numbers. There is no digital signal above 0dbfs, so there is no need to have positive numbers. 0dbFS is the abosolute limit to the level of signal that can be expressed digitally.
Line level (the level every piece of equipment was designed to run at) = 0dbVU = -20dbFS.
It is a little more complicated than that, but these are the basics