Network virtualization allows multiple isolated virtual networks to run simultaneously on a shared substrate infrastructure to provide diversifying services to the end user, solving the Internet ossification problem. However, a major challenge is efficiently mapping multiple virtual networks with different topologies into a shared infrastruture, named the virtual network embedding problem. This paper surveys the current literature primarily according to the composition of the infrastructure. Firstly, the concept and feature of network virtualizations are elaborated, and the corresponding model of virtual network embedding is formulated. Secondly, the latest research progress of virtual network embedding algorithms is reviewed according to the composing way of infrastructure, the integrity of problem space, the number of embedding stage and so on. Finally, the potential future research directions are outlined in the aspects of fairness, scalability, high utilization and trust.