To make the replay deterministic, the study presents the definition of VM replay by constructing a VM execution model, and then proves the sufficient conditions of VM replay using formal expressions of the algebra. Based on these conditions, the paper presents CASMotion, a Xen based implementation of VM execution replay. CASMotion classifies the category of non-deterministic events in Xen domU and presents their replaying methods and time matching algorithms. The experiment results show CASMotion can accurately replay the non-deterministic events with low performance penalty.