Configuring yum to keep more than three kernels
When you run ‘yum update’ on your Fedora system, the default yum configuration will keep the last 3 kernels. This allows you to fail back to a previous working kernel if you encounter an error or a bug. The number of kernels to keep is controlled by the installonly_limit option, which is thoroughly described in the yum.conf(8) manual page:
installonly_limit Number of packages listed in installonlypkgs to keep installed at the same time. Setting to 0 disables this feature. Default is ’0′. Note that this functionality used to be in the “installonlyn” plugin, where this option was altered via. tokeep. Note that as of version 3.2.24, yum will now look in the yumdb for a installonly attribute on installed packages. If that attribute is “keep”, then they will never be removed.
If you need to keep more than 3 kernels, you can increase the value of installonly_limit in /etc/yum.conf.