  We report on the field dependence of the microwave complex resistivity data
in YBa$_2$Cu$_3$O$_{7-x}$/BaZrO$_3$ films grown by PLD at various BaZrO$_3$
content. The data, analyzed within a recently developed general framework for
the mixed-state microwave response of superconductors, yield the field
dependence of the fluxon parameters such as the vortex viscosity and the
pinning constant. We find that pinning undergoes a change of regime when the
BaZrO$_3$ content in the target increases from 2.5 mol.% to 5 mol.%.
Simultaneously, the vortex viscosity becomes an increasing function of the
applied magnetic field. We propose a scenario in which flux lines are pinned as
bundles, and a crossover from dilute point pins to dense c-axis correlated
defects takes place between 2.5 and 5 mol.% in the BZO concentration. Our data
are inconsistent with vortices occupying mainly the BaZrO$_3$ sites at low
fields, and suggest instead that vortices occupy both BaZrO$_3$ sites and
interstitials in the YBa$_2$Cu$_3$O$_{7-x}$ matrix, even at low fields.
