In Cloud computing, virtualization is the basis of delivering Infrastructure as a Service (IaaS) that separates data, network, applications and machines from hardware constraints. Although Cloud computing has been a focused area of research in the last decade, research on Cloud virtualization security has not been extensive. In this paper, different aspects of Cloud virtualization security have been explored. Specifically, we have identified: i) security requirements for virtualization in Cloud computing which can be used as a step towards securing virtual infrastructure of Cloud, ii) attacks that can be launched on Cloud virtual infrastructure, and iii) security solutions to secure the virtualization environment by overcoming the possible threats and attacks.