Infrastructure as a Service (IaaS) provides logical separation between data, network, applications and machines from the physical constrains of real machines. IaaS is one of the basis of cloud virtualization. Recently, security issues are also gradually emerging with virtualization of cloud computing. Different security aspects of cloud virtualization will be explored in this research paper, security recognizing potential threats or attacks that exploit these vulnerabilities, and what security measures are used to alleviate such threats. In addition, a discussion of general security requirements and the existing security schemes is also provided. As shown in this paper, different components of virtualization environment are targets to various attacks that in turn leads to security issues compromising the whole cloud infrastructure. In this paper an overview of various cloud security aspects is also provided. Different attack scenarios of virtualization environments and security solutions to cater these attacks have been discussed in the paper. We then proceed to discuss API security concerns, data security, hijacking of user account and other security concerns. The aforementioned discussions can be used in the future to propose assessment criteria, which could be useful in analyzing the efficiency of security solutions of virtualization environment in the face of various virtual environment attacks.