I was failed reboot for load openvz kernel because (maybe)kernel couldn't found LVM's volume. That said :
ALERT! /dev/mapper/ubuntu--vg-root does not exist. Dropping to a shell!
I was trying to found /dev/mapper directory in dropped shell but there is no mapper directory.
So, After booting from original kernel, edit /boot/grub/grub.cfg file for change that soft link filename(/dev/mapper/ubuntu--vg-root) to UUID. But still failed because there is no LVM volume's UUID in /dev/disk/by-uuid when dropped to emergency built-in shell.
Tutorial seems to be something missing. Any idea?