A hypervisor has a much smaller attack target than a full blown OS. The kernel alone of a mainstream OS can be an order of magnitude larger than a hypervisor. Xen, for instance, has fewer than 150,000 lines of code. The Linux kernel has about 15 million. A full Linux distro probably has around 200-300 million. So a full OS has about two orders of magnitude (100x) more potential for exploits.