After performing updates to the virtual machine, the vm kernel panics. Hit space on the boot screen and select the old kernel. Then open a terminal and do the following substituting your kernel version.
mkinitrd --with=hid-base-hv --with=hid-hyperv --with=hv_utils --with=hv_vmbus --with=hv_storvsc --with=hv_netvsc /boot/initrd-2.6.18-371.8.1.el5.img 2.6.18-371.8.1.el5 -f
From: http://community.spiceworks.com/topic/486741-kernel-panic-after-migrating-centos-5-10-to-hyper-v
