Hydraulic Valve

....and done....

http://hydraulicspneumatics.com/hydraulic-valves/what-difference-between-servovalve-and-proportional-valve

In what context? The more information in an original post, the quicker and more concise will be the answer.

Try again. This time, try using several sentences to give some meaning and context to the question.

In fact from the basic function none!

The proportional valve is a servovalve with not so high a frequency limit, with not so high linearity and not so small leak and which requires for control a higher current.

Servovalves were developed first because servohydraulics were first thought for planes when the force to move the different control surfaces became too big and for ground armored vehicles when the turret was too heavy and the necessity to turn it fast became too stringent. Because of the "quality" servovalves were and are expensive.

About 35 years ago servohydraulics entered the industrial domain and since for many applications the servovalve was too expensive and offered more than needed a new valve was conceived : the proportional valve.

It must be said that today's proportional valves have properties which are in some cases even better than some of former servovalves. Most of the progress is due to the evolution of PM which offer a much higher force density than the classical ALNICO.