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Energy-efficient optical control of photonic device properties is crucial for diverse photonic signal processing. Here, we demonstrate extremely efficient optical tuning of photonic nanocavities via the photorefractive effect, with only photon-level optical energy. With a lithium niobate photonic crystal nanocavity with an optical Q up to 1.41 million and an effective mode volume down to 0.78(λ/n)^3, we are able to achieve a resonance tuning efficiency of about 88.4 MHz/photon (0.67 GHz/aJ), which allows us to tune across the whole cavity resonance with only about 2.4 photons on average inside the cavity. Such a photon-level resonance tuning is of great potential for energy-efficient optical switching, wavelength routing, and reconfiguration of photonic devices/circuits that are indispensable for future photonic interconnect.