Cloud computing is an emerging computing paradigm that may change the way how information services are provisioned. Network virtualization plays a crucial role in a Cloud environment for abstracting and virtualizing various network infrastructures as services. Therefore virtual network services should be integrated with Cloud service to form the composite Cloud service. However, little research has focused on modeling and optimization of network virtualization in Cloud service provisioning to end users. In this paper, we model the Cloud service provisioning feature in virtualization-based Cloud computing, and propose an exact algorithm for QoS-aware service composition to optimize user's experiences for Cloud service access. Our theoretical analysis indicates that the proposed algorithm is light-weighted and cost-effective. We also compare the proposed algorithm against a variant of the best-known QoS routing algorithm experimentally. The results show that the proposed algorithm has better performance both in execution time and finding solution. We believe that the modeling technique and the algorithm presented in this paper are general and effective, thus are applicable to practical Cloud computing systems.