If you are running CentOS Linux 7 inside a Xen DomU (VM) in PV mode, you can not upgrade to the standard 7.4.1708 kernel that is currently in the CR repo and that will be soon released in our file 7.4.1708 tree
(3.10.0-693.*el7)

The CentOS Plus kernel will be available when we release the 7.4.1708
tree and it does boot inside a Xen DomU PV, so you can either upgrade to the CentOS Plus kernel in PV mode .. or you can shift your DomU to either HVM or PVHVM modes.  PVHVM is more like PV mode, and is usually much better than HVM mode.

More info on PVHVM here:
https://wiki.xen.org/wiki/PV_on_HVM

https://wiki.xen.org/wiki/Xen_Linux_PV_on_HVM_drivers

The anaconda installer and pxe booting from the standard tree 7.4.1708
tree will also not work in PV mode for new installs, only PVHVM or HVM
installs can be done with the 7.4.1708 tree.

Note:  This is an upstream source code issue and will not be fixed in the main CentOS Linux 7 kernel unless it is fixed upstream in the RHEL
source code.

We are sorry for any inconvenience this issue may cause. Thanks, Johnny Hughes