Network virtualization has emerged as one of the strong technologies to enable an organization to optimize resource utilization for higher scalability and operational efficiency in networking environments. Network virtualization allows the abstraction of physical network resources as logical units to create multiple virtual networks on a single physical infrastructure. This technological advancement invites several security challenges that may nullify the very advantages that technology aims to ensure. This paper systematically reviews some of the most critical security challenges associated with network virtualization, including multi-tenancy risks, data isolation, vulnerabilities of hypervisors, and threats during VM migration. The review also integrates prior literature on classic security frameworks that cannot respond to the unique risks introduced by virtualized environments. It will analyze methodologies for the mitigation of such security threats and also provide recommendations to help improve security measures in network virtualization. This study provides an overview of the current status of challenges to network virtualization security and identifies areas for future research in a quest for a more secure networking landscape.