Kilovolt-amps measure what’s called the ‘apparent power’ of a generator. This is different from kilowatts (kW), which measure the ‘true power’. The difference between the two is the power factor (PF), which is given as a ratio or percentage.
You can calculate kVA into kW by multiplying with the power factor. The PF is usually 1 for single-phase generators, which means kVA is the same as kW. For three-phase generators, the PF is usually 0.8.
That means a 20 kVA three-phase generator has a true power of 16 kW (20 x 0.8 = 16).