I am using services such as AWS and MS Azure to host websites for my clients. Is it OK / Legal to use those companies logos on my websites as a partner or this is considered infringement?
As a partner, no. This implies two-way traffic. I mean, it might be legal depending on where you are and which companies, but it's confusing enough that I'd consider it unethical.
But it's very clear they are simply tools to use and/or service providers.
Using logos is permitted so long as you are not trying to imply something false, like a relationship. Thus, news articles get to use logo's since they are writing about a company, and if there is a relationship they typically will make some disclosure somewhere in the article.
Partner has special meaning in many jurisdictions. Just because you are their customer, it does not make you their partner.
Why would you want to use someone else's logo? If you are trying to let their fame rub off on you, this is exactly what most providers don't want you doing - if you were somehow to become unstuck (prosecuted for fraud for example) it would be to their cost/detriment for their name to be associated with you or your business.
Some companies have rules that permit you to use their logos as they it as free marketing/promotion, others have rules that strictly prevent usage. You will need read the small print of your contract.