1. A method for classifying activity levels for memory pages of a virtual machine, the method comprising: receiving notification of an elapse of one of a plurality of supported time intervals; identifying a current activity level for a memory page of the virtual machine stored in a data structure maintained for the elapsed time interval; retrieving input data indicating an activity level of the memory page during the elapsed time interval; determining a new activity level for the memory page based on the current activity level and the retrieved input data; and storing the new activity level in an entry of the data structure corresponding to the memory page.