You can prevent yum command from updating the Kernel permanently by following the simple steps.
Option #1: Edit /etc/yum.conf file
Use a text editor such as vi to edit /etc/yum.conf:
# vi /etc/yum.conf
Append/modify exclude directive line under [main] section, enter:
Save and close the file. Try, updating the system without updating the Linux kernel:
# yum -y update
This is a permanent option, so you don't need pass the -x option to yum command.
Option #2: Pass the -x option to prevent yum from updating kernel
The syntax is as follows to skip update on command line itself:
# yum -x 'kernel*' update