I'd like to calculate the performance ratio of a hybrid system with photovoltaic.  Therefore I need the solar radiation on the PV array, that means the solar radiation incident on the PV array in kWh. In the hourly data of the simulation results I have the opportunity to plot the 'Global Solar Radiation Incident on the PV Array Monthly Averages'. But this graphic shows the data in kW/m². Where can I change it into kWh?

HOMER correctly denominates the radiation incident on the PV array in kW/m2.  You will have to multiply by the surface area of the panel in m2 (HOMER does not know this) and by a time interval to find the kWh of solar radiation that strikes the panel in that time interval.