Paris/Italy 15-day itinerary suggestions

Just a note that on your days April 27, 29, and especially May 1, you will not really get 1/2 day in each of the two cities + travel time. Depending on what time your flight/train leaves, you get half a day in probably ONE of those cities (either on the front or back end), but only a couple of hours in the other. That is, you might get for example on 4/27 a morning in Paris, but then by the time you get to Venice and get settled it will be late afternoon, so you really only have the evening. This may be fine for what you want to do, just be aware that those transfers (and the accompanying pack up, check out of hotel, get to airport/train station, find your way, do the travel, get oriented upon arrival, get to your hotel etc) can consume more time than you might think.

You don't state specifically, but it appears you're flying back out of Rome. Is that the case? If so, good thinking not to backtrack back to Paris.

May 1 is a holiday in Europe, their labor day. Italian public transport might be operating on a holiday schedule which may or may not slow you down a bit. (It would in Paris, but not enough to really make too much of a difference; just something to be aware of.)

At first glance it appears that Venice, Florence, and the area south of Rome are being "short changed". And since it almost always takes longer than anticipated to get from place to place, you might want to scale back the number of places you go. You really want to enjoy the places you visit as opposed to just having a blurry memory of them. It appears you have 15 nights total. Perhaps just do Paris, Venice, Florence and Rome this trip (and even that might be rushing) and leave south of Rome for another trip. Or maybe leave off Florence. Just one person's opinion.

Personally, I think you are trying to hit too many places, especially in Italy. I think you have allotted Paris and Rome sufficient time but I don't think you have enough time in Venice. Florence or the Amalfi Coast region. My suggestion would be to cut one of those locations out and spend more time in the remaining 3. Check skyscanner.net for flights between Paris and Venice. Air France has multiple flights on April 27th ranging from $56 to $145 and the flight is 1h 40m. Taking into account transit time to the airport in Paris as well as the time it takes to get from the airport to your hotel in Venice, you will easily spend 6 hours in transit that day. The train from Venice to Florence is a shade over 2 hours and from Florence to Naples is 2 1/2 to 3 hours, depending on which train you take. The fast train between Naples and Rome is a little over an hour. Based on these travel times, I think you are really only allocating a day and a half in Venice and Florence. If you do not want to see the museums, this is probably OK. You can hit the highlights but won't be able to delve too deeply into the art scene.

You probably already know this, but if you know your travel dates/times, you can save some $$ buying train tickets in advance. Check both trenitalia.com and italotreno.it for fares. Both have high speed trains but the schedules are different and sometime the price is vastly different for the same route on the same day. I believe you can buy tickets either 120 days in advance or 90 (i don't remember which so check 120 days out and see what happens!). Word of caution - when you buy the train tickets, make sure you sign up for an account on their website. I have seen a lot of messages here asking how to retrieve tickets because they did not receive the e-mail with their print-at-home tickets. With an account, you can retrieve your tickets anytime, anyplace.

The others have said it better than I, but I agree: to really get into these places, you would do better to leave out one place.

Actually, I think your itinerary looks good and very feasible. Yes, April 27 might be a bit crazy and you may not have as much time as you think in Venice for that day bc of flights and transfers. However, you can enjoy your first evening in an enchanting city. I have not been to the Amalfi Coast. Another option for those days would be to rent a car at the Florence airport and do a bit of touring of Tuscany, returning the car in Orvieto and then taking the train into Rome. Just another alternative to all of the city siteseeing you are doing. Enjoy your trip.

As others have said, you really shouldn't count your travel days as a possible half day somewhere.
I would want extra time in Venice and Florence, perhaps at the expense of Capri and one of your days in Rome.
April 23 -- Paris (4 nts)
April 24 -- Paris
April 25 -- Paris
April 26 -- Paris
April 27 -- Fly to Venice (3 nts)
April 28 -- Venice
April 29 -- Venice
April 30 -- Train to Florence (3 nts)
May 1 -- Florence
May 2 -- Day trip to Siena
May 3 -- Train to Naples then Sorrento (2 nts) (or a 4th night in Florence)
May 4 -- Morning to Pompeii & afternoon to Amalfi Coast (or train to Rome for 4 nts)
May 5 -- Train to Rome (3 nts)
May 6 -- Rome
May 7 -- Rome

I also agree, too much traveling around. I would consider---

April 23 - 25 -- 3 nights in Paris.

April 26 - 28 -- 3 nights in Venice

April 29 - May 1 -- 3 nights in Florence, with one full day in Siena

May 2 - May 3 -- 2 nights in Sorrento, day trip to Amalfi Coast. Skip Capri

May 4 - May 7 -- 4 nights in Rome (or do 4 nights Paris/3 nights Rome)

Just my opinion, but you will enjoy whatever you choose!