Set in /etc/default/grub:
...
###GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0,115200n8 console=tty1"
GRUB_CMDLINE_LINUX=""
...
GRUB_TERMINAL="serial console"
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
Refresh grub using
grub2-mkconfig -o /boot/grub2/grub.cfg # CentOS, RHEL
or
update-grub2 # debian (et al)
RHEL 6 based distributions, like Scientific Linix 6 or CentOS 6 use the older legacy (v1) grub, where the user edits /boot/grub/grub.cfgdirectly. To get grub to use the serial console for displying the menu, the config file should contain, before the hidemenu line:
splashimage=(hd0,0)/grub/splash.xpm.gz
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
terminal --timeout=5 serial console
:hiddenmenu
To activate the serial kernel console, for booting and shutdown messages, the kernel comand line should end in
... rhgb console=tty0 console=ttyS0,115200n8
instead of
... rhgb quiet
This will automatically activate the getty program on the serrial console to provide the login prompt on the serial console.
Systemd based distributions start a getty process for the serial console automatically when they detect the serial console option on the kernel command line. It should NOT be necessary to activate the serial console manually.
For reference, the command to activate the serial console manually is
systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service
Your VM has to have a serial console configured. Make sure the console configuration of the VM is set to serial, not virtio for if you want to be able to use the console for grub and in single user mode. Verify that you see something like
<console type='pty'>
<target type='serial' port='0'/>
</console>
when looking at the output of virsh dumpxml vm-name. The important setting is the type attribute of the target tag. Other tags and atributes can be set.
Using the serial type for the console, you get the usual /dev/ttyS0 console. Using virtio, you get /dev/hvc0 as the console device. Most modern linux distributions automaticallt start a getty process when they detect the presence of a serial console. To detect the presence of a serial console, the kernal command line has to contain console=ttyS0, wheares a virtio console is detected by the presence of /dev/hvc0. This means you can get a serial console to a fully booted system without any additional configuration of the client machine when you use the virtio driver.
To get kernel output to the serial console during booting or in case of failures, you need to activate the serial console on the kernel command line. This can be done with either driver. It seems that there is no module in grub for the virtio driver. This means you have to use the serial driver to be able to use the serial console to pick or edit entries from the grub boot menu.
Bottom line: use virtio if you only care about console login to a working system and your distribution automatically starts a getty process on /dev/hvc0. For full control for the boot process and to be able to use the serial console to debug and fix problems booting, use the serial driver and configure grub and your kernel as described above.
Remote administration tools like IPMI provide access to a serial console. This is a minimal console, which uses less network bandwitdh than the graphical console provided by remote admin tools. Also, the remote graphical console often requires Java or proprietory tools. The serial console will often use a different serial console, e.g., you might have to substitue ttyS1 (or a different number) for ttyS0 in the examples listed above. With ipmitool, the command access to connect to the serial console on a remove server looks something like
ipmitool -H remote-ipmi-host -U remote-user -LOPERATOR -I lanplus sol activate