I've travelled to paris solo twice before. I even walked into a sketchy neighborhood once (I was just trying to explore beyond the tourist sites to try to find some great photos or a cafe off the beaten path), but never felt unsafe. I've even stayed out until 2 AM there (on Bastille day). The key is to just look like you know where you're going, even if you're lost and then find a hotel or some place that could provide directions (if you're lost- works for me all the time). It also helps if you look European and don't make the dumb mistake of wearing something that says where you're from- I laugh each time I see someone wearing such garb. This is the case for me, and literally everywhere I've been, everyone thinks I'm a local! Too funny!
Someone tried to pickpocket me in notre dame (unsuccessful because I had my purse locked in btwn my arm and body)! So watch out in crowded touristy areas and metro! But that's really all tag happened there- no other incidents.
All of the places you listed are safe (although some parts of Italy have some problems, depending on where you're going.