I guess it's a little confusing.
You can't control a current without varying the voltage and you can't control a voltage without varying the current. However for most solenoid type valves, the relationship between current and valve position may be close to being very linear (proportional) as shown in that little graph in the datasheet.
With PWM you basically control the average voltage and thus also the average current, however you may find that the relationship between PWM duty cycle and valve position may not be as linear (proportional) as current control. You may need to do a little experimenting to find what PWM values result in what valve position.