There is a growing demand for provisioning of different levels of quality of service (QoS) on scalable Web servers to meet changing resource availability and satisfy different client requirements. The proportional differentiation model is getting momentum because of its fairness and differentiation predictability. It states that QoS of different traffic classes should be kept proportional to their pre-specified differentiation parameters, independent of the class loads. In this paper, we present a processing rate allocation scheme for providing proportional response time differentiation on Web servers. A challenging issue is how to achieve processing rates for different request classes in the implementation. We propose a process allocation strategy, which dynamically and adaptively changes the number of processes allocated for handling different request classes while ensuring the ratios of process allocation specified by the processing rate allocation scheme. We implement the process allocation strategy at application level on Apache Web servers. Experimental results show that the processing rate can be achieved by the adaptive process allocation strategy and the Web servers can provide predictable and controllable proportional response time differentiation.