Q:

What's the difference between `nfs-server` and `nfs-kernel-server`? 

Autocompletion shows
$ sudo service nfs- stop
nfs-blkmap         nfs-common         nfs-config         nfs-idmapd         nfs-kernel-server  nfs-mountd         nfs-server         nfs-utils          

What's the difference between nfs-server and nfs-kernel-server?  Do I need to start both and stop both like
$ sudo service nfs-server start

$ sudo service nfs-kernel-server start

$ sudo service nfs-server stop

$ sudo service nfs-kernel-server stop

A:

The only difference is their name; nfs-kernel-server is an alias for nfs-server:
$ systemctl show -pNames nfs-server
Names=nfs-server.service nfs-kernel-server.service

Starting and stopping either one will start and stop the NFS server.