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I would like to understand the technical difference between Public and Private blockchain. I understand the public blockchain by referring Ethereum tutorial. But I didn't understand the private blockchain architecture, How the node setup? who's controlling the network? Who decides the genesis block?

There are a lot of resource on Google to find yourself deep explanations


Blockchain Architecture Analysis: Private vs Public vs Consortium
https://hackernoon.com/blockchain-architecture-analysis-private-vs-public-vs-consortium-65eb061b907b

Types of Blockchains & DLTs (Distributed Ledger Technologies)
https://blockchainhub.net/blockchains-and-distributed-ledger-technologies-in-general/

In-depth on differences between public, private and permissioned blockchains
https://medium.com/@lkolisko/in-depth-on-differences-between-public-private-and-permissioned-blockchains-aff762f0ca24

Google told me there are 20 700 000 résults

The structure of the private blockchain can be pretty much the same as the structure of the public blockchain. However, what's different are the access rights: everyone can access a pubic blockchain, but you need special privileges to access a private blockchain.

For example, you can send your transactions to a public blockchain, but you may not be able to do that with a private blockchain. You many not be able to run a node unless given access rights, etc.

So, in a way, a private blockchain is the antithesis of what blockchain technology is trying to achieve in the first place (decentralization).

What's the point then? One may argue that private blockchains are faster as the nodes are centralized and may be run by a firm or organization. But you are trusting and relying on a centralized entity, which defeats the purpose of this technology. So, IMO, private blockchains are totally missing the point.

eBay’s security breach 2014 was the beginning of a series of cyber attacks to enterprises from inside via their own employees and one of the serious security concerns that exist in centralized databases is data manipulation.

What a private blockchain could offer here, is a database that is highly sensitive to data manipulation. In the case of a data manipulation by an untrusted employee in a common centralized model, you just have a field of data that could simply change and detection process of such scrambled data is quietly hard. But in private blockchain you somehow have a summarized hash root that could simply get verified by end user.

"Blockchain", although being the current buzzword associated with Bitcoin and cryptocurrencies, is actually just a particular data structure.  Using that data structure may make certain business processes more efficient or secure, so there may be some use for "private blockchains".

However, as has already been pointed out, the real revolution with Bitcoin is actually not just the use of a blockchain, but the clever combination of various things (including a blockchain but also PoW mining to solve double spending).  This combination enabled completely new things, namely a fully decentralised and trustless money system.  So while a blockchain as data structure can be useful, it is IMHO not a complete game changer in a private setting.  The real break-through invention is using it in a permission-less setting like Bitcoin.

That's not necessarily true. I can think of a million ideas to implement a private blockchain, where it would actually be useful. Imagine a "decentralised" government transaction ledger. Where not everyone has access, but for example every county/department has rights to broadcast a tx and view the blockchain.

Or for example a blockchain inside a specific country, where a node can only have a local IP, which can be used for any solution that requires citizenship of that country.

Don't even get me started about businesses and multi-national industries. I could genuinely go on forever. Private blockchains aren't useless. You don't need to make something completely public, to decentralize it. If you make it decentralized among a controlled amount of people, it's still decentralized.

You have a good point there. But then, the question still remains: what about trustlessness and immutability in such environments. Can we trust the government employees, or business with vested interests that they will remain impartial while running their blockchain?

It should be pretty clear that the private party controls the network and decide the protocol rules. Not everyone can participate in the private blockchain to verify the transactions.
The term "public and private blockchain" often considered different from "open and closed blockchain" or "permissioned and permissionless blockchain" but in my opinion, they are pretty much the same.

These blockchains have a very specific use case, unlike the open/permissionless/public one. Maybe consortiums of banks or companies will use it to smooth their operations.

As far as I know there's only one project with private chains ready to roll and integrate with the public chain and that's NEM.

The private version is this -

https://nem.io/enterprise/mijin/

https://mijin.io/en/  

Perhaps 'persmissioned' is a better bit of terminology than private.

It's not PoW which may make a difference, but maybe the prime difference in actual operability will be the ability of the controller to select the infrastructure to run it. A permissioned chain can choose to be carefully tuned to maximise performance.

In terms of NEM Mijin can run up to several thousand tps. The public equivalent will max out around 100.

Even though private blockchain are centralized, this doesn't mean they are missing the point.

Private blockchain still share  some interesting atributes of the public ones, such as immutability, transparency, historical record, traceability..

Some companies which work with confident information may use private blockchain to protect and store their data. Some other day I was in a conference at work where I discovered that big companies are considering using blockchain technology as a database.

Blockchain technology is here to stay. I believe the decentralization is a  Bitcoin important atribute, as a currency.but blockchain technology have some other applications which do not require a full decentralization

In a private blockchain Network the you will need to obtain an invitation to join the network. Sometimes because of this reason it is also called as permission Network since you need a permission to join the network. The existing participants have the control to decide the future participants . All these participants are responsible to maintain the blockchain in the same decentralized manner.

Nothing helps understand blockchains better than building one yourself.

In the end it all boils down to "control"

The Blockchain can still be decentralized <having multiple nodes being run from different countries or locations>, but what is most important is, Who is in control of those nodes and the Blockchain network.

A Bank can for example decentralize their own private Blockchain by running full nodes at all of their branches globally, but one entity is controlling all those nodes. <So they will have a more secure network, but they control everything>

A public Blockchain gives access to everyone and it is controlled by the majority consensus decision making model and not by a single entity.  ;)