is there an XML standard to describe a component's characteristic
Sure there's a standard.  There's 14 of them - pick the one you want or create standard #15. As kyle said you can define the part information in terms of a specific piece of software's XML format but that doesn't buy you anything in general.

The best you can do is create something that works for your workflow.  That might mean working entirely within one piece of software's format (Eagle, TinyCAD, KiCad, etc.) or defining your own format and creating an XSL transform to change the XML into something that can be used by Eagle, etc. (Situation: there are 15 competing standards).