This paper investigates the application of passivity-based nonlinear control to the problem of primary voltage stabilization in medium-voltage DC microgrids (MVDC mGs) given by the interconnection of nonlinear distributed generation units (DGUs) and power lines. To this aim, we propose nonlinear local regulators which steer the voltage at the output terminal of each DGU to a reference value. Each controller can be explicitly synthesized relying on DGU parameters, voltage reference values of the neighboring DGUs and resistance of the neighboring power lines. The control design enables plug-and-play (PnP) operations: a plug-in or -out of a DGU requires only the update of regulators of neighboring DGUs without spoiling the stability of overall mG. Theoretical results are backed up by simulations in Simulink environment.