This paper presents a generic model called TGV for efficient evaluation of XQuery in a heterogeneous distributed system. XQuery is a rich and so a complex language that allows users to express a large scale of queries over XML documents. This expressiveness makes difficulties to obtain an exclusive internal representation within a system. To this purpose, models based on Tree Patterns have been proposed: TPQ [ameryahia01], generalized by the GTP [chen2003]. However, they do not capture well all the expressivity of XQuery, cannot handle mediation problems, and do not support extensible optimisation and sources information.. We present a tree pattern-based model called TGV that - integrates the whole functionalities of XQuery - uses an intuitive representation that provides a global visualization of the request in a mediation context - provides a framework for extensible optimization using a rules definition model - take into account all knowledges useful for the query evaluation (cost model, accuracy, etc.) This work has been implemented in the XLive system \cite {dangngoc2005} based on a full-XML architecture.