Fixed displacement pumps
Used when a generator with a PTO (power take off) is available fixed displacement pumps give a singe (fixed) flow at a given engine RPM.
These are the traditional type of hydraulic pump installation and are favoured when long runtimes or intensive usage is needed.
Highly robust these systems are relatively expensive & require a proper engineering feasibility study before each installation.
 

Variable displacement pumps
Used in conjunction with a DC “Power Valve” power pack in an integrated hydraulic system, a variable displacement pump offers variable flow capacities and a multitude of pressure settings.
A variable displacement pump can also be used when a fixed flow is needed but the engine RPM is likely to vary. This would typically be a main engine or gearbox mounted pump.

In both cases the motor used to run the pump needs to have a least 30% more power available at the RPM used, than the power rating of the thruster it is supposed to run.
Variable displacement pumps are also more costly than both fixed flow and DC pumps.