Network virtualization involves creating multiple virtual net-works within a physical network, based on the exible net-work environment of software-defined networking (SDN). In a virtualized SDN (vSDN), a network hypervisor plays a key role in translating physical network resources into vir-tual network resources. However, this translation in vSDN incurs considerable overhead in network monitoring that is critical for network management. In this paper, we pro-pose Flo-v to provide network monitoring in a vSDN. Flo-v provides accurate monitoring information to a vSDN and, by applying selective and adaptive approaches, it monitors traffic in a network hypervisor with lower overhead than ex-isting schemes. We compare the proposed Flo-v with exist-ing schemes, and demonstrate that Flo-v provides accurate network information with reduced resource usage in terms of both the network and CPU, with approximately 50% less overhead than existing schemes.