How can keep output voltage constant in pv system with MPPT (FOCV method) and buck converter?
Generally, the output voltage in a solar charging system is held constant by the battery itself. For a first order model, the output of your system is connected to a voltage souce! The MPPT regulates the input voltage, without even considering the output voltage, much as a traditional voltage regulator regulates the output voltage and doesn't care what the input voltage is.

Of course, a complete (and safe) charger design should include secondary loops or controls to disable charging when the battery is charged all the way up, and also possibly to limit the current into the battery if the input power may exceed the maximum charge rate of the battery.

Edit: I should clarify that, as Tony details in his response, there are many implementations of MPPT, some of which might monitor output voltage and current as part of their sophisticated slow-loop that optimizes the bias point of the cell. However, the feedback for the primary DC-DC converter loop is generally at the input. Unsophisticated MPPT solar chargers simply use a pre-programmed input regulation voltage that is reasonably optimal for the expected conditions.