Будьте осторожные при обновлении CentOS до релиза 7.4 (1708) с ядром 3.10.0-693 в виртуальной среде Xen PV. В Xen HVM проблемы нет.

• Workaround for CentOS 7 Xen VM with grub2 and Xen dom0 with pv-grub
• Install CentOS Plus kernel (kernel-plus)
• Migrate pv-grub on Linode VPS
• CentOS Plus Kernel for Xen
• Upgrade with caution: CentOS 7.4 (1708) does not support PV

<panel type="danger" title="CentOS-7 (1708) Release Notes"> 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. Please see this mailing list thread for more details. </panel>

Осталось попробовать заменить grub2 на grub-legacy https://lists.centos.org/pipermail/centos/2015-August/154261.html

Yeah just yum erase grub2 and then force the installation of the CentOS 6 grub package; then run grub-install.