8051 is a microcontroller without any analog input so you need an external A/D converter. Of course the 8051, the display and the A/D converter will be powered by 5Vdc. To measure DC voltages and currents, you need external precision resistors connected as voltage divider to reduce input high voltage to the value correct for the A/D converter (usually 5V or less). You need extra circuits (active precision rectifier) to measure AC voltages and currents.
Take note that usually, in multimeters, the voltage under measure is not insulated from the microcontroller circuit so if you measure 220Vac, your circuit must be well insulated to avoid electrical shocks.