Generator rating

Generator is producer of Voltage and Current, we can connect load as such draining full load current or half load, even noload, but voltage remain moreover same. So it will rated in terms of product of V*I, secondly it gives a electrical output for any Load(may Capacitor, Inductor & resistor),so it is consider as Product of Voltage & Current excluding power factor. Whereas motor is machines coverts electrical Energy to Mechanical energy, so it is rated in KW 

Generator rating is given in kVA,as generator is the souce  and delivers power to load. Load may be resistive, inductive  or capacitive. A 10 MVA generator will deliver 10 MW load if  power factor is unity,however if the power factor of the load is 0.8 then generator can deliver active power up to 8MW only. It is up to user who efficienly generator power  may be utilised. Therfore, manufacturer always specify the  generator rating in KVA or MVA. On the other hand, motor converts electrical energy into mechanical energy. Therfore,its rating is given in KW as power factor of the motor remain constant. 

The rating of generator is described in terms of KVA by its manufacturer because the generator will produce a constant voltage on which it is designed and the current will depend on the nature of load, for same load it delivers more current for poor power factor and less current for unity power factor, since the manufacturer does not know the nature of load he described its rating in terms of voltage and current. While in case of motor, motor have fixed power factor. 

We know that,

                     KW=KVA*PF