What's included in Linode's external kernels?

I'm new to Linode. I just started an instance running Ubuntu 15.04. The kernel right now is "3.19.1-x86_64-linode53". Apparently this is what is called an "external" kernel in the Xen-speak, instead of the stock kernel?

The "/lib/modules/3.19.1-x86_64-linode53/" directory is basically empty, which means there are probably a lot of kernel modules compiled into the external kernel? So how do I know what's included in this external kernel?