Typical Hydraulic Servo Setup (as used in structural testing):
The power source is usually a pump (e.g. a swash-plate pump) arranged to provide (up to a rated flow rate) at ~constant pressure. Hydraulic accumulators may be included to smooth out transient pressure drops.
The valves are typically directional control valves, so they can drive a piston/cylinder both ways. See https://en.wikipedia.org/wiki/Directional_control_valve
For structural testing, the valves are usually piloted spool valves, electrically (coil) driven, precision ground (more often, lapped.) These valves are fairly linear (in terms of coil-current-input to flow-rate output) and impressive bandwidths (hundreds of Hz., esp. in flight controls like aircraft, missiles or booster rockets) but that linearity comes at the cost of flow leakages, on the order of a couple percent of the peak flow (per valve!) The valve drive is often dithered (strongly!) to minimize stiction (e.g. from particles in the fluid getting into the valve.)
Feedback transducers used vary, but for high-power/high-damage-if-anything-goes-wrong systems, analog sensing, especially built-right-into-the-device, ratiometric sensing (e.g. LVDTs) is useful, because such analog stroke feedback permits simple, robust (e.g. PID) control loops for position control. Load control is also common for systems with a reliable force sensor; cylinder loadcells are common.