Which countries are supported by PayPal payments?

We would like to receive payments worldwide, also from developing countries.
Can we use PayPal for this?

It depends on the country.
PayPal is often a cheap solution (especially for smaller events) to receive payments from the US, Canada, Australia, the UK etc.

However, some developing countries are not supported, as there were probably too many problems with fraud (at least that is what we suppose) or the market is not profitable enough for companies like PayPal.
Please note that some acquirers don't accept credit card payments from such countries, either.
Therefore, the idea "we need credit card payments" is often not the solution for all payment problems.

The list of countries supported by PayPal can be found here (there are really only few exceptions):
https://www.paypal.com/worldwide/
For countries not supported you should consider bank transfer and cash on arrival.

Countries currently not supported are (no liability assumed):
Antarctica, Afghanistan, Cote D'ivoire, Cuba, Belarus, Bouvet Island,
British Indian Ocean Territory, Cameroon, Central African Republic,
Christmas Island,  Equatorial Guinea, Haiti, Heard Island and Mcdonald Islands,
Iran, Iraq, North Korea, Laos, Lebanon, Liberia, Libya, Myanmar,
Nigeria, Pakistan, Papua New Guinea, Serbia and Montenegro,
Sudan, Syria, Turkey, Zimbabwe.

Another option to receive card payments from (nearly) anywhere is Stripe.
You find an overview of countries where this service is available for organizers on this page:
https://stripe.com/global
But this list is not for clients, they can pay from any country that is not on a ban list for international money transactions.
Unlike PayPal, participants never need an account, they just enter their card details after the registration process.

Finally you may also check if Skrill (formerly Moneybookers) is an option for you.
However, their fees have increased significantly during the last years, therefore we have only few clients using Skrill.
The fees charged by Skrill for merchant accounts can be viewed here:
https://content.skrill.com/en/business/merchant-fees/

It's no problem to use more than one service with ConfTool.