dbm is milliwatt while adding V to the end is millivolt.
Watt and voltage are a lot different.
dBmV. This unit is used in television receiver systems in which the system
impedance is 75 ohms, rather than the 50 ohms normally used in other RF
systems. It refers to the signal voltage, measured in decibels, with respect
to a signal level of one millivolt (1mV) across a 75 ohm resistance (0 dBmv).
In many TV specs, 1 mV is the full quieting signal that produces no "snow"
(i.e. noise) in the displayed picture.
0dBm = 48.75 dBmV.
The dBmV is a unit of signal power, not voltage, because the impedance is
held constant at 75 ohms.
kthaddock