Early transition metals (TMs) of the 3d and 4d rows are undesired contaminants in solarandelectronic-grade Si. From the theoretical standpoint, understanding the properties of these TMs insilicon still remains a challenging problem owing to the strong correlations among the TM d-electrons.The present study proposes a first-principles Hubbard-corrected DFT+U approach, with on-site parametersaccounting separately for electron Coulomb (U) and exchange (J) effects. We use this approachtogether with conventional DFT to determine electrical levels and migration barriers of early3d (Ti, V and Cr) and 4d (Zr, Nb and Mo) TMs in Si. Comparisons with experimental data allowedus to uniquely assign the deep levels in the gap appraising also the effect of on-site correlation. Ourresults also resolve existing controversies in the literature concerning the type and origin of the donorlevels of Cr and Mo. For all the metals, with the exception of Cr, high barriers of interstitial diffusionare obtained, thus confirming that most of these TMs are slow diffusers in silicon.