How is ATP converted to adenosine?
For a full understanding I refer you to the KEGG pathway map which is fairly comprehensive in showing all the metabolic processes that CAN produce Adenosine. 
It's a bit daunting but if you find it on the map here it is interactive and you can see the enzymes and pathways involved. If you drill down it brings you to a host of papers and references in primary literature to back things up. 

However the dominant process is as follows:

   2 ADP &lt;=&gt; ATP + AMP  


enacted by an adenylate kinase then:

   AMP + H2O &lt;=&gt; Adenosine + phosphate


enacted by adenosine 5'-monophosphate phosphohydrolase (a 5'-Nucleotidases )