In modern stars, the more massive a star is, the more likely that it is powered by the CNO fusion cycle, where Carbon, Nitrogen, an Oxygen act as catalysts for creating helium from hydrogen. In stars over 1.3 solar masses, this is the primary fusion process. Below that that level it's a mix of the CNO and P-P cycles. P-P is supposed to be much slower than CNO, since it uses no catalysts. Since neither Carbon, Nitrogen, nor Oxygen existed during the Pop III epoch, were all of these massive stars powered simply by the P-P cycle? Well at least entirely P-P until they produced the CNO internally eventually?