DAV stands for “Distributed Authoring and Versioning”. RFC 2518 defines a set of concepts and accompanying extension methods to HTTP 1.1 that make the web into a more universal read/write medium. The basic idea is that a WebDAV-compliant web server can act like a generic file server; clients can “mount” shared folders over HTTP that behave much like other network filesystems (such as NFS or SMB.)
The tragedy, though, is that despite the acronym, the RFC specification doesn't actually describe any sort of version control. Basic WebDAV clients and servers assume only one version of each file or directory exists, and can be repeatedly overwritten.
Because RFC 2518 left out versioning concepts, another committee was left with the responsibility of writing RFC 3253 a few years later. The new RFC adds versioning concepts to WebDAV, placing the “V” back in “DAV” — hence the term “DeltaV”. WebDAV/DeltaV clients and servers are often called just “DeltaV” programs, since DeltaV implies the existence of basic WebDAV.
The original WebDAV standard has been widely successful. Every modern computer operating system has a general WebDAV client built-in (details to follow), and a number of popular standalone applications are also able to speak WebDAV— Microsoft Office, Dreamweaver, and Photoshop to name a few. On the server end, the Apache webserver has been able to provide WebDAV services since 1998 and is considered the de-facto open-source standard. There are several other commercial WebDAV servers available, including Microsoft's own IIS.
DeltaV, unfortunately, has not been so successful. It's very difficult to find any DeltaV clients or servers. The few that do exist are relatively unknown commercial products, and thus it's very difficult to test interoperability. It's not entirely clear as to why DeltaV has remained stagnant. Some argue that the specification is just too complex, others argue that while WebDAV's features have mass appeal (even the least technical users appreciate network file-sharing), version control features aren't interesting or necessary for most users. Finally, some have argued that DeltaV remains unpopular because there's still no open-source server product which implements it well.
When Subversion was still in its design phase, it seemed like a great idea to use Apache as a network server. It already had a module to provide WebDAV services. DeltaV was a relatively new specification. The hope was that the Subversion server module (mod_dav_svn) would eventually evolve into an open-source DeltaV reference implementation. Unfortunately, DeltaV has a very specific versioning model that doesn't quite line up with Subversion's model. Some concepts were mappable, others were not.
What does this mean, then?
First, the Subversion client is not a fully-implemented
DeltaV client. It needs certain types of things from the server
that DeltaV itself cannot provide, and thus is largely dependent
on a number of Subversion-specific
REPORT requests that
only mod_dav_svn understands.
Second, mod_dav_svn is not a fully-realized DeltaV server. Many portions of the DeltaV specification were irrelevant to Subversion, and thus left unimplemented.
There is still some debate in the developer community as to whether or not it's worthwhile to remedy either of these situations. It's fairly unrealistic to change Subversion's design to match DeltaV, so there's probably no way the client can ever learn to get everything it needs from a general DeltaV server. On the other hand, mod_dav_svn could be further developed to implement all of DeltaV, but it's hard to find motivation to do so—there are almost no DeltaV clients to interoperate with.