We have developed a fully cryogenically cooled phased array feed (PAF) prototype camera and tested it on the Arecibo Radio Telescope in the summer of 2013. The camera consists of 19 dual-polarized elements on a hexagonal grid. The array elements are modified sleeve dipoles specially designed to be reinsertable, to include a noise injection coupling port and to operate at 18 K. Experimental results for observations on the Arecibo Telescope demonstrate performance of the cryogenic amplifiers, digital beamforming, and sensitivity over the PAF field of view, and provide an important proof of concept for a full-scale PAF receiver.