In this report, we investigate the security aspects and challenges about computer virtualization. In a few words, virtualization is what allows the execution of multiple operating systems on a single machine at the same time. A virtualization component can be viewed as a layer or a container making some kind of emulation, allowing to execute programs or operating systems on the virtualized layer, for example executing Microsoft Windows and Linux on one single machine at the same time. On one hand, as virtualization can provide a kind of isolation between users/applications/operating systems, it can address some security containment problems. But on the other hand, there exist today many security flaws and attacks focused on such systems, as the virtualization layer controls and monitors all virtualized applications/operating systems. The outline of this report is the following. We first review precisely all the various forms the virtualization layer and related components can take. And what technical aspects it involves. Then we survey more precisely all the vulnerabilities and exploits that currently exist or may appear in the near future. And then we study what are the proposed security protection by actual virtualization technologies. We conclude the report with what is remaining to be done and what are the forthcoming security challenges.