This document summarizes Xen security framework (XSM) which enables fine-grained control over interactions between domains, hypervisor, and resources. XSM uses mandatory access control based on security labels rather than discretionary access control. Permissions for subjects (processes or VMs) to interact with objects (files, ports, devices, etc.) are defined in security policies. The architecture includes security policies, a policy controlling entity, security server, access vector cache, and policy database. The decision making process involves checking the access vector cache, consulting the security server and policy database if needed, and returning the access decision. Challenges include ensuring atomic policy changes and consistency between security policy and runtime policy database.