Since you have asked this question five years ago, things have changed and more people from Fachhochschulen are doing PhDs (although you still have to have a supervisor from a proper university). I know many people who do a joint PhD, where they stay at the Fachhochschule and do their research there (most of the time applied research), but are enrolled as a PhD student at a proper University. The main issue is that many of the disciplines that one can study at a Fachhochschule differ from the disciplines that are tought at proper universities, so finding a supervisor can be challenging, as their specialty might be quite far from the aspiring PhD students area of study. Thus, it is quite common that the PhD candidate from the Fachhochschule needs to take several additional courses. In general, if you have really good grades and are determined to do your PhD, is is perfectly possible to do it even with a Fachhochschul-Master, albeit it will probably take a it more effort than for those coming from a proper university.

There are efforts to give even (select) Fachhochschulen the so called "Promotionsrecht", allowing professors from Fachhochschulen to supervise the PhD candidates on their own, without aonther professor from a proper university necessary.

One thing to add is, that since some years, professors at Fachhochschulen are requiered to have a PhD themselves (that was not the case until a few years back). Because of this, some Fachhochschulen have difficulties in finding appropriate candidates (as people with the right, applied skills often don't have a PhD) and often professor posts are vacant as a result. So as a Fachhochschul-PhD holder, it might be easier to attain a professorship as they are in high demand.

Since you have asked this question five years ago, things have changed and more people from Fachhochschulen are doing PhDs (although you still have to have a supervisor from a proper university). I know many people who do a joint PhD, where they stay at the Fachhochschule and do their research there (most of the time applied research), but are enrolled as a PhD student at a proper University. The main issue is that many of the disciplines that one can study at a Fachhochschule differ from the disciplines that are taught at proper universities, so finding a supervisor can be challenging, as their specialty might be quite far from the aspiring PhD students area of study. Thus, it is quite common that the PhD candidate from the Fachhochschule needs to take several additional courses. In general, if you have really good grades and are determined to do your PhD, is is perfectly possible to do it even with a Fachhochschul-Master, albeit it will probably take a bit more effort than for those coming from a proper university.

There are efforts to give even (select) Fachhochschulen the so called "Promotionsrecht", allowing professors from Fachhochschulen to supervise the PhD candidates on their own, without aonther professor from a proper university necessary.

One thing to add is, that since some years, professors at Fachhochschulen are requiered to have a PhD themselves (that was not the case until a few years back). Because of this, some Fachhochschulen have difficulties in finding appropriate candidates (as people with the right, applied skills often don't have a PhD) and often professor posts are vacant as a result. So as a Fachhochschul-PhD holder, it might be easier to attain a professorship as they are in high demand.