Create a new configuration, tell xbuild to use it:
In Visual Studio, create a new configuration that excludes the projects you're not interested in. -(Build -> Configuration Manager..., select on the Active solution platform: drop down list)
Using the Configuration Manager, remove unwanted solutions from the configuration.
Next, close VisualStudio, which will save changes to the .sln and .csproj The .sln will record which projects are associated with the configuration. The .csproj will record the settings of the configuration, such as whether TRACE or DEBUG is defined.
Finally, when calling xbuild assign your configuration's name to the Configuration property.
E.g.
xbuild /p:Configuration="NoUnitTests" ServerResource.sln
The above will build the projects associated with the NoUnitTests
configuration.