Efficient energy use has become a critical issue for designing and managing of cloud data centers. Virtualization is a key technology for reducing energy cost and improving resource utilization in data centers. One of the challenges faced by virtualized data centers is to decide how to pack VMs on the least number of physical machines. This paper presents a VM management framework which is based on a multi-agent system to minimize energy consumption and Service Level Agreement (SLA) violations. The proposed agents are arranged in a three level hierarchical structure to perform VM assignment, VM placement and VM consolidation in a data center efficiently. Experimental results demonstrate that the framework achieves high quality solution in spite of its simplicity and scalability.