They are different metering standards, each referenced to a different thing. A dB is only a ratio of one signal to another, with 0dB being no difference between the two.
dBFS = Decibels full scale, always a negative number relating to how much digital headroom you have left.
dBVU = Decibels volume units, another type of metering usually found on analog equipment, especially mixers. Most mixers' meters are calibrated so when the signal reads 0dBVU on the meter it is +4dBu, or 0.775 volts RMS, at the output. Many Mackie mixers are calibrated so +4dBVU = +4dBu.
dBSPL = Decibels sound pressure level, a measure of acoustic energy in sound traveling through the air.