XML database for linux
I don't know much about graph database system and their capabilities but several traditional (SQL) database systems have options to store and manipulate XML documents:


PostgreSQL: there is a XML data type and quite a lot of XML functions since version 8.3.
SQL Server: has support for XML Data since version 2012 (and as announced this year, the DBMS will be soon available in Linux and not only Windows).
Oracle: XML DB Developer's Guide, I think since version 9.
DB2: offical docs: pureXML overview -- DB2 as an XML database and a nice article that explains its XML capabilities: DB2 Native XML.