Niobium thin lms are used at CERN (European Organization for Nuclear Research) for coatings of superconducting radio-frequency (SRF) accelerating cavities. Numerical simulations can help to better understand the physical processes involved in such coatings and provide predictions of thin lm properties. In this article, Particle-in-Cell Monte Carlo (PICMC) 3D plasma simulations are validated against experimental data in a coaxial cylindrical system allowing both DC diode and DC magnetron operation. A proper choice of ion induced secondary electron emission (IISEE) parameters enables to match experimental and simulated discharge currents and voltages, with argon as the process gas and niobium as the target element. Langmuir probe measurements are presented to further support simulation results. The choice of argon gas with a niobium target is driven by CERN applications, but the methodology described in this paper is applicable to other discharge gases and target elements. Validation of plasma simulations is the rst step towards developing an accurate methodology for predicting thin lm coatings characteristics in complex objects such as SRF cavities.