The reduced frequency regulation capability in low-inertia power systems necessitates enhanced frequency support from photovoltaic (PV) systems. However, the regulation capability of PV system under conventional control scheme is limited, which requires flexible power control and support from battery energy storage systems (BESSs). This paper proposes an energy management strategy of PV-BESS to provide stable frequency support to the grid. The proposed strategy initially develops a maximum power point tracking (MPPT)-based power reserve control (PRC) for PV power reserve. At this stage, the BESS is manipulated to buffer the power fluctuations caused by MPPT execution and environmental variability. To enhance the regulation capability of BESS and mitigate battery aging, the proposed strategy incorporates a flexible PRC for state of charge (SOC) recovery. Subsequently, the battery can be rationally charged or discharged during the PRC process to maintain SOC. Simulation results obtained from MATLAB/Simulink and experimental results from the StarSim platform validate that the proposed strategy achieves stable PV power reserve, mitigates fluctuations induced by environmental uncertainties, enables efficient SOC recovery, and improves grid frequency quality. Overall, the proposed strategy ensures stable operation of PV-BESS both under steady-state conditions and during frequency events.