There is not always a difference between these. The two types of discipline can overlap to a great degree. However, the basic difference is that positive discipline is, by definition, more aimed at prevention of problems before they happen while progressive discipline tends to punish violations after the fact.
A progressive discipline system is one in which the severity of discipline increases with each infraction. It lays out in writing all the penalites that will be imposed on the employee for each type of violation. It is one in which an employer builds a record of having disciplined at each infraction so that it will not get into legal trouble if it has to severely discipline or terminate an employee.
By contrast, a positive discipline system is one that tries to prevent problems by maintaining high morale. It is one that seeks a great deal of employee input in devising ways to prevent problems from arising. It is much less formal than a progressive discipline system.