The thermal growth of silver nanoparticles (Ag NPs) embedded in Poly-vinyl alcohol thin films is studied as a function of annealing time by in-situ spectroscopic ellipsometry in the visible spectral range. Each recorded spectrum is analyzed by the Shape Distributed Effective Medium Theory model from which are determined the time evolutions of the film thickness, the shape distribution, the volume fraction of the nanoparticles and the effective dielectric function of the nanocomposite film. The estimated shape distribution shows that the nanoparticles remain almost spherical in the course of the annealing process in good agreement with the electronic microscopy examinations of the samples. As expected, the films exhibit a plasmon resonance band, the amplitude of which increases during the annealing while the film thickness decreases. The growth mechanism of the silver NPs was also investigated by analyzing the variation of their volume fraction in the films. The obtained results show that the NPs growth occurs via a mechanism involving Ag atoms (monomers) additions and/or reduction of Ag ions directly onto the AgNPs surface.