Polymer Electrolyte Membrane Fuel Cells (PEMFC) are extremely sensitive to load variations. A rapid load variation can damage the system in many ways. For this reason, a Load Governor (LG) based on constrained Extremum Seeking (ES) approach has been proposed in this paper. The LG controller ensures that the physical constraints of the system are not violated and the ES controller varies the LG bandwidth parameter to optimize the net power output. The proposed controller has been tested on a Hardware-In-Loop (HIL) emulation test bench, based on a commercial centrifugal compressor and a real time PEMFC emulator. Experimental results show a good performance of the proposed scheme during load variations.