Domain value maps are a way of storing lookup values in SOA server such that these lookup values can be easily edited at run time using SOA console, shared across composites and stored in MDS. Creation of MDS is done using Oracle JDeveloper and once it is deployed onto the SOA server, its values can be easily modified and managed using SOA console.
Java embedding activity is a means of embedding Java and J2EE code directly inside Oracle BPEL process. Even though java embedding can provide enhanced functionality (since you can plug in any java code here!!), this should be avoided as much as possible – since the code would be harder to manage and will not be “SOA” like. Since this is an Oracle extension, it is not part of BPEL standard spec – hence won’t be portable!!