  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 TN-NTNs integration 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. In this survey, we provide a comprehensive review on
the adaptation of these networking paradigms in 6G networks. We begin with a
brief overview on NTNs and virtualization techniques. Then, we highlight the
integral role of Artificial Intelligence in improving network virtualization by
summarizing major research areas where AI models are applied. Building on this
foundation, the survey identifies the main issues arising from the adaptation
of SDN, NFV, and network slicing in integrated TN-NTNs, and proposes a taxonomy
of integrated TN-NTNs virtualization offering a thorough review of relevant
contributions. The taxonomy is built on a four-level classification indicating
for each study the level of TN-NTNs integration, the used virtualization
technology, the addressed problem, the type of the study and the proposed
solution, which can be based on conventional or AI-enabled methods. Moreover,
we present a summary on the simulation tools commonly used in the testing and
validation of such networks. Finally, we discuss open issues and give insights
on future research directions for the advancement of integrated TN-NTNs
virtualization in the 6G era.
