In this article, virtualization security concerns in the cloud computing domain are reviewed. The focus is toward virtual machine (VM) security where attacks and vulnerabilities such as VM escape, VM hopping, cross-VM side-channel, VM-based rootkits (VMBRs), VM mobility, and VM remote are mentioned and discussed according to their relevance in the clouds. For each attack we outline how they affect the security of cloud systems. Countermeasures and security measures to detect or prevent them through techniques such as VM detection, GuardHype, VM introspection, VM image scanning, etc. are also discussed. Through the surveyed work we present a classification of VM threats within the clouds. Finally, we include our observations and those of other researchers on this matter of cloud virtualization security.