dBu are decibles referenced to 0.775 volts, where 0 dBu corresponds to 1 mW of power in a 600 ohm load.
dBv are decibles referenced to 1V, where 0 dBv corresponds to 1V.
Pro audio equipment references 0 dB to +4 dBu, which is 0.775V * 10 ^ (4/20), or 1.228 volts.
Consumer audio equipment references 0dB to -10 dBv, which is 1.000V * 10 ^ (-10/20), or 0.316 volts.
Thus the references are about a factor of 3.8 apart (or 11.8 dB), which makes interfacing pro and consumer audio gear somewhat tricky.