To quote forum member OPEN DES directly from another thread regarding the A320:
"FPV is actually a derivative of Vzbi. Vzbi is baro-inertial vertical speed, i.e.: the one we see indicated in normal conditions.
FPA is derived using GS and Vzbi.
In conditions where Vzbi is not available FPA is derived from the difference between pitch and AoA, this is however a degraded FPA, relative to the air only"