Do you wang to know?OK,I’ll tell you.
Let’s see the details about them firstly.
XviD

XviD codec produces industry-leading compact high quality MPEG-4 video. Its quality and efficiency has made it one of the most popular video codecs online. The XviD codec is developed and maintained by a handful of skilled and interested engineers from all over the world. Playback of XviD movies is usually supported in new DVD players nowadays. This codec makes it possible to compress a full-length DVD-quality movie enough to fit on a single CD, while still maintaining high image quality. The video is usually combined with MP3 or AC3 audio.
Microsoft Windows Media Video 9

Microsoft's video codec makes vast improvements over that from series 8, and more importantly, the decode stream parameters are now fixed ?future iterations of Windows Media Video will make improvements in the encoder, but those will be able to play back on devices that are WMV9 compatible. This has enabled them to find support in several new and a great many upcoming DVD players, digital media adapters, and portable video players
DivX

The codec from DivX, Inc is popular online, and has even found its way into several consumer electronics devices. DivX ;-) 3.11 alpha refers to a hacked version of the Microsoft MPEG-4 Version 3 video codec, extracted around 1998 by French hacker Jerome Rota (also known as Gej). In early 2000, Rota created a company to improve the codec development. Free versions of DivX Pro before 5.2 typically contained spyware. From 5.2 onwards, including version 6.4, no spyware was included, but Pro version (with DivX encoder) is only available in the form of a paid release
SUMMARY:
We can  compar many examples using WMV9, Xvid and DivX codecs, and came to a conclusion that video created with XviD codec has the best quality.Of course, quality isn't the only factor when it comes to personal video encoding.If you need,you can use some tools(such as ImTOO MPEG Encoder) to convent between mepg wmv and divx or any other formats.
Generally speaking, The best codecs are XviD and Windows Media Video 9 -- it all depends on your priorities. If you're going to stick to computers as playback devices, the faster speed and excellent image quality of XviD is welcome. If you want to play your stuff on the PDAs, portable video players, and DVD players, WMV9 has broader industry support and is worth the extra encoding time. Both codecs delivered quite impressive image quality.
That’s all.Have you got the idea?

here is something about XviD,which seems very same as the Divx ,but in fact, they have many differences between them, sounds very interesting !

XviD An open source MPEG-4 video codec project which was launched in 2001 to continue Project Mayo's open source DivX codec.

Basically a short piece of history is required in here to understand this fully.

Original DivX codec was not developed from the scratch, but was just a hacked version of Microsoft's WMV video codec. This version, best known as DivX v3.11 alpha (which was the last official version of the hacked codec), quickly became ridiculously popular among videofreaks all over the world. 

After its success, group of people, most notably the hacker behind the DivX codec, started developing a legal version of the codec which would be a real codec instead of basic hack of existing Microsoft codec. 

They launched an open source project, dubbed as Project Mayo (also known as OpenDivX) to develop this codec. After a while the codec matured, but the original developers wanted to push it further and started a company called DivXNetworks which started developing its own closed source version, based on the work of the Project Mayo.

Eventually this closed source codec was released as DivX 4 (since then, we've seen this closed source version to develop further and currently, 09/2005, the latest version of the codec is called DivX 6.0).

you know whats even more amazing?  I just ran across some movies I downloaded years ago, such as Crouching Tiger Hidden Dragon. It's a long movie, with plenty of movement. I downloaded this on a 56k, and it's only 440MB for the whole 3 hour movie.  It was encoded with the mp43 codec: S-Mpeg 4 version 3.  And considering the movie length and file size the quality is terrific.