1. A method for balancing virtual machine loads comprises: monitoring load information of each of physical hosts and virtual machines operating on the physical hosts; calculating a load index of each of the physical hosts and the virtual machines operating on the physical hosts respectively according to the load information; based on the load index of each of the physical hosts, determining a source physical host on which any virtual machine needs to be migrated so as to reduce the load index of the source physical host, wherein determining the source physical host comprises at least one of selecting a physical host with a largest load index as the source physical host, selecting a physical host with a load index larger than a first preset threshold as the source physical host, and selecting a physical host whose load index is always larger than the first preset threshold in a preset number of recent monitoring measurements; determining a target virtual machine to be migrated on the source physical host based on the load index of each of the virtual machines on the source physical host and respective affinity factor indicating a degree of dependency of a virtual machine on a physical host on which it resides; and migrating the target virtual machine into a destination physical host different than the source physical host.