KVA = Kilo Volt Amperes
Power factor = Efficiency
KW = Kilo Watts = KVA x PF
It's a bit complicated to put it into simple terms but I will try.
Power factor is the relationship (phase) of current and voltage in AC electrical distribution systems. Under ideal conditions current and voltage are "in phase" and the power factor is "100%." If inductive loads (motors) are present, power factors less than 100 % (typically 80 to 90 % can occur)
Low power factor, electrically speaking, causes heavier current to flow in power distribution lines in order to deliver a given number of kilowatts to an electrical load.
As the loads on an aircraft generator are varied and cause phase shift, the efficiency suffers. Reactive/inductive loads cause heat losses. Therefore the REAL power of the generator in kilo-watts will be less than the design rating in kilovolt amperes.
I'm sure this is clear as mud but it's the best I can do.