The standard Echomap has, I believe, just a traditional type sonar, so will display ground, obstacles and fish arcs. The DV models have the DownVu high definition sonar which covers a much narrower beam but in very high detail and gives images that look a bit like an ultrasound scan. Google DownVu images and the monochrome ones are the HD scan, the multicolour ones are traditional sonar. I find DownVu better for spotting dive sites, but fish show up as dots and can be hard to spot initially. The traditional sonar gives less detail but fish arch are easier to spot than dots.
I think the 45DV has both traditional and DownVu, but don't quote me on that.