An In-Depth Survey on Virtualization Technologies in 6G Integrated Terrestrial and Non-Terrestrial Networks

6G networks are envisioned to deliver a large diversity of applications and meet stringent Quality of Service (QoS) requirements. Hence, integrated Terrestrial and Non-Terrestrial Networks (TN-NTNs) are anticipated to be key enabling technologies. However, the integration of TN-NTNs faces a number of challenges that could be addressed through network virtualization technologies, such as Software-Defined Networking (SDN), Network Function Virtualization (NFV), and Network Slicing (NS). In this survey, we provide a comprehensive review of the adaptation of these networking paradigms in 6G networks. We begin with a brief overview of Non-Terrestrial Networks (NTNs) and virtualization techniques. Then, we highlight the integral role of Artificial Intelligence (AI) in improving network virtualization by summarizing major research areas where AI models are applied. Building on this foundation, we identify the main issues arising from the use of SDN, NFV, and NS in integrated TN-NTNs, and propose a taxonomy of integrated TN-NTNs virtualization offering a thorough review of relevant contributions. The taxonomy is built on a four-level classification that indicates — for each study — the level of TN-NTNs integration, the virtualization technology used, the problem addressed, the type of the study, and the proposed solution, which can be based on conventional or AI-enabled methods. Finally, we discuss open issues and give insights on future research directions for the advancement of integrated TN-NTNs virtualization in the 6G era.