The goal of Web Designer is to offer an easy to use, uncluttered tool specifically for creating web pages, websites and web graphics.
Xara Photo & Graphic Designer (formerly called Xtreme) is intended more for print graphics, illustration and photo work. If you own Photo & Graphic Designer you will find it provides a good solution for creating individual web graphics, but if you intend to do a lot of web page or website design then you might want to consider adding Web Designer or upgrading your Photo & Graphic Designer to Designer Pro.
Designer Pro X (v8) is our top of the range product and includes everything in Web Designer MX Premium v8 and Photo & Graphic Designer MX 2013, plus additional pro design features (such as Pantone, color separations)
Note: April 2013. We have just released Web Designer 9 Premium. The new web features & templates of v9 are not yet in Designer Pro, they will be added to Designer Pro X9 when it is released.