<p>Innovations in new applications and technological advancements are driving the evolution of network architectures towards flexibility and automation. Network Function Virtualization (NFV) deploys Network Functions (NFs) as software applications onto cloud infrastructures, redefining the development, deployment, and operation models of communication networks, thereby meeting the evolution demands of networks. However, after more than a decade of development, the progress of network service operators in NFV has not met expectations, partly because some key technologies remain unresolved. To accelerate the large-scale commercial use of NFV, this paper focuses on reviewing relevant literature from the past five years. Based on practical applications and insights into future trends, we explore the three directions of network virtualization, network cloudification, and network service orientation. We investigate the most representative technologies and the latest research progress in these fields, analyze the current problems and challenges, and provide corresponding suggestions on how to deal with them. Finally, we forecast future directions of technological development.</p>