I am not very knowledgeable when it comes to cryptocurrency but I happened to have attended a lecture on discord where these two items were talked about and some of their differences between the two is what I will listing here.
First and foremost, the first and most important different is *IPOs work well when its centralized and fully control by a corporation while ICO works well when it is open source meaning no central authority.
ICO is the creation of digital tokens on public ledger called block chain while IPO is like a distribution of shareholding and this is done to the public via investment banks they call them underwriters.
ICO may not have product to present to the public but they have proof of stake or proof of work but IPO must provide a product.
I hope these differences are enough to answer your question.