If you are running CentOS 7 on PV you will not be able to boot after an upgrade to CentOS 7.4.

Yesterday, Wednesday 13 September, CentOS 7.4 (1708) was released. While this is only a minor release (with CentOS 7.3 having been released in August 2017), the release notes contain a critical erratum:

If you are running CentOS-7 as a Xen domU in ParaVirtualization (PV) mode, an upgrade to CentOS-7 (1708) will cause the VM to not be able to boot. You must use HVM (full emulation) or PV-on-HVM mode to run this version of CentOS with the Xen hypervisor.

You can find discussion of this issue on CentOS-Virt.

To determine whether you are running a PV system, log in to the management console. If you have a legacy console you are on a PV system. On the latest management console, the system details (option ‘a’) screen will display: “Virtualization mode: PVM” if you are running PV.

You will need to upgrade to an HVM VPS to run CentOS 7.4. For those wishing to self-service this migration, you will need to order a new HVM VPS. We will give you up to a month’s credit while you migrate your PV system to HVM. You can migrate your existing IP address by writing support@prgmr.com before you cancel your PV VPS. We can also migrate an existing PV VPS in-place if you’re confident in your ability to replace pv-grub with grub. You will need to change your serial console from hvc0 to ttyS0.

If you wish to continue running PV on CentOS 7.4 you will need to run an alternate kernel. The CentOS Plus kernel works. Here are step by instructions on how to install it:

yum remove $(rpm -qa | grep kernel-3.10.0-693)
yum install yum-utils
yum-config-manager --save --setopt=centosplus.enabled=1
yum-config-manager --save --setopt=centosplus.includepkgs='kernel*'
yum-config-manager --save --setopt=base.exclude='kernel*'
yum-config-manager --save --setopt='updates.exclude=kernel*'
yum update
yum install kernel-plus
# For grub2
/sbin/grubby --grub2 --set-default=$(ls /boot/vmlinuz*centos.plus*)
/sbin/grubby --grub2 --default-kernel # verify this has centos.plus in it
# For pv-grub
/sbin/grubby --grub --set-default=$(ls /boot/vmlinuz*centos.plus*)
/sbin/grubby --grub --default-kernel # verify this has centos.plus in it

If you have already upgraded and your host will not boot, you may be able to boot using your previous kernel using grub. Grub can be accessed via option 1 (out of band console) on your Management Console.

If you have any questions are need more information, please write support@prgmr.com.