To explain the difference lets turn to Rainscreen Cladding: A Guide to Design Principles and Practices (Anderson, Gill).
“The main point to note about the drained and back-ventilated approach is that claddings are allowed to leak, and no deliberate attempt is made to minimize the effects of wind by means of pressure equalization. Instead, the cavity behind the cladding is drained and positive back ventilation is used to promote the rapid evaporation of any rainwater deposited on the inner leaf. The same process is used to evacuate the water vapour which permeates through the inner leaf and, if present, it’s insulating layer.”
By contrast, Anderson and Gill further offer the following differentiation to PERS:
“The essential defining attribute that differentiates PERS from D/BV is the design and use of compartmentalization within the cavity to allow pressure equalization that accounts for the constantly changing wind pressures across the face of any building. This makes these systems much more design intensive."