The default is to have 2 kernels versions.
The version you are running with and the previous version.
When a new kernel is installed it removes the oldest one. A reboot starts the new one and (if it doesn't work) you can reboot, hit esc key and select the old one.
There are several different kernels for each version.
the vinilla called kernel
the XEN - if you installed XEN packages.
and PAE which is for a 32-bit system that can use > 4GB
So you will get the kernel and PAE kernel together
the XEN kerenl will update slower.
But you will only have 2 version of each (unless you install one from an RPM - that will not remove the older ones.
The next update would remove 2 older ones.
I you are not going to use PAE or XEN then
yum remove them
that will save you 30 MBs of downloading / updated kernel.