Kate (KDE Advanced Text Editor) is my favourite editor for programming in any language. It is a multi-view editor that lets you view several instances of the same document with all instances being synced, or view more files at the same time for easy reference or simultaneous editing. The terminal emulation and sidebar are docked windows that can be plugged out of the main window, or replaced therein according to your preference.
Some random features:
Editing of big files
Extensible syntax highlighting
Folding
Dynamic word wrap
Selectable encoding
Filter command
Global grep dialog
Install kate and kate-plugins (or kde-addons).
Open your XML file.
Enable the Kate XML Completion plugin-in at
->A new menu, titled XML, will show up. Select
-> and choose /usr/share/apps/katexmltools/kde-docbook.dtd.xml