Abstract: Cloud computing is a pay-as-you-go resource usage schema.It provides the quality of service(QoS) guaranteed computing,storage and the network resources via the virtualization technologies.However,there are some difficulties for virtual machine QoS assurance during the process of resource provision due to the virtualized environments.For example,it is limited by the virtualization technologies themselves,and difficult to predict the behavior of applications run on virtual machines.Meanwhile,it is difficult to predict the performance of virtual machines and also difficult to monitor their behaviors.Our work first elaborates on these difficulties,and then analyzes the state-of-the-art studies on the QoS assurance of virtual machines.Finally,we summarized the QoS related problems of virtual machines.We further discuss the future research issues and critical problems that need to be solved.