The source of the geothermal energy is the heat from the magma. The geothermal energy is commonly found in the places that are near the plate boundaries, namely the convergent plate boundaries. Where there's volcanic activity, in the surrounding area almost always there's also geothermal energy to be found. Because there's lot of cracks in the crust in this type of places, the magma manages to generate its heat through them, so the water in the crust is heated. Because there's also very high pressures as well, the water is very often coming in the form of geysers, thus shooting upwards few meters in height.