I. Introduction
A. Hypervisors: From Virtual Machines to Virtual Networks
B. Combining Network Virtualization and Software Defined Networking
C. Scope of this Survey
D. Contributions and Structure of this Article

II. Background and Related Surveys
A. Network Virtualization
B. Software Defined Networking

III. Virtualizing SDN Networks
A. Managing a Physical SDN Network with a Hypervisor
B. Network Attribute Virtualization
C. Isolation Attributes

IV. Classification Structure of Hypervisor Survey
A. Type of Architecture
B. Hypervisor Execution Platform

V. Centralized Hypervisors
A. FlowVisor
B. General Hypervisors Building on FlowVisor
C. Hypervisors for Special Network Types
D. Policy-based Hypervisors

VI. Distributed Hypervisors
A. Execution on General Computing Platform
B. Computing Platform
C. Computing Platform
D. Computing Platform

VII. Summary Comparison of Surveyed Hypervisors

VIII. Hypervisor Performance Evaluation Framework
A. Background: Benchmarking Non-virtualized SDN Networks
B. Benchmarking SDN Network Hypervisors

IX. Future Research Directions
A. Service Level Definition and Agreement for vSDNs
B. Hypervisor Reliability and Fault Tolerance
C. Hypervisor Performance Benchmarks
D. Hardware and Hypervisor Abstraction
E. Scalable Hypervisor Design
F. Hypervisor Placement
G. Hypervisors for Special Network Types
H. Self-configuring and Self-optimizing Hypervisors
I. Hypervisor Security

X. Conclusion