What’s the difference between `nfs-server` and `nfs-kernel-server`

As Noufal Ibrahim says, I think this is a Solaris convention.

IIRC, /export/home is used on the server where the actual files live, and /home is where the other servers mount it.

What does mount | grep home say? I'm guessing that /export/home has a file system type of UFS, and /home has a type of NFS?

/etc/fstab may also have some clues.

unfs3 is dead as far as I know; Ganesha is the most active userspace NFS server project right now, though it is not completely mature.

Although it serves different protocols, Samba is an example of a successful file server that operates in userspace.

I haven't seen a recent performance comparison.

Some other issues:

• Ordinary applications look files up by pathname, but nfsd needs to be able to look them up by filehandle. This is tricky and requires support from the filesystem (and not all filesystems can support it). In the past it was not possible to do this from userspace, but more recent kernels have added name_to_handle_at(2) and open_by_handle_at(2) system calls.
• I seem to recall blocking file-locking calls being a problem; I'm not sure how userspace servers handle them these days. (Do you tie up a server thread waiting on the lock, or do you poll?)
• Newer file system semantics (change attributes, delegations, share locks) may be implemented more easily in kernel first (in theory--they mostly haven't been yet).
• You don't want to have to check permissions, quotas, etc., by hand--instead you want to change your uid and rely on the common kernel vfs code to do that. And Linux has a system call (setfsuid(2)) that should do that. For reasons I forget, I think that's proved more complicated to use in servers than it should be.

In general, a kernel server's strengths are closer integration with the vfs and the exported filesystem. We can make up for that by providing more kernel interfaces (such as the filehandle system calls), but that's not easy. On the other hand, some of the filesystems people want to export these days (like gluster) actually live mainly in userspace. Those can be exported by the kernel nfsd using FUSE--but again extensions to the FUSE interfaces may be required for newer features, and there may be performance issues.

Short version: good question!