Ubuntu 14 server edition, how to boot into single-user mode?

I got the answer. At this line:

linux /vmlinuz-3.13.0-32-generic root=/dev/mapper/ubuntu-vg-root ro

append single and it becomes to

linux /vmlinuz-3.13.0-32-generic root=/dev/mapper/ubuntu-vg-root ro single

Then press Ctrl + x. You will goes into single user mode.

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To make this permanent, you need to edit /etc/default/grub and change this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to

GRUB_CMDLINE_LINUX_DEFAULT="text"

The details may vary depending on your system. The important bit is setting GRUB_CMDLINE_LINUX_DEFAULT to text. Once you have done this, run sudo update-grub and next time you restart you will boot into text mode.