A method for zeroing guest memory of a VM during boot up, includes the guest OS attempts to set a page to zero. A page fault is generated and is handled by the hypervisor. The page is mapped by the hypervisor to a page in host memory, and is given to the guest. The guest OS attempts to set the next page to zero. Another page fault is generated the hypervisor unmaps the host memory page, and the second page is mapped to the same page. The hypervisor then gives the page to the guest, which contains all zeros. The process is repeated for remaining pages of the guest memory.